Dockerfile Essentials: Building Docker Images from Scratch


Developers who are new to Docker may feel overwhelmed by the seemingly endless possibilities of containerization. Dockerfiles provide a simple, straightforward way to create custom images that can be used across different environments, making it an essential tool in the containerization process. In this article, I will provide an overview of Dockerfile essentials and guide you through the steps of building customized images from scratch.

Definition of Dockerfile

A Dockerfile is a text file that contains a set of instructions for building a Docker image. It is essentially a recipe that tells Docker what steps to take when creating an image from scratch or updating an existing one.

The instructions in a Dockerfile are executed in order, and each instruction creates a new layer on top of the previous one. Dockerfiles use plain text files with no special formatting requirements.

Each instruction starts with a keyword followed by its arguments. The most commonly used keywords include FROM, RUN, CMD, COPY and EXPOSE.

The FROM keyword specifies which base image to use as the starting point for building your own image. RUN executes commands within the container while it’s being built and allows you to install dependencies and configure settings necessary for your application.

Importance of Dockerfile in building Docker images

Dockerfiles allow developers to automate the process of creating custom images with their applications or services installed on top of base images provided by vendors or community members. By using predefined layers specified in existing base images as well as adding their own customizations through each step listed in their corresponding dockerfiles via command-line tools like pip or npm, developers can ensure consistency across deployments.

Dockerfiles also make it easy for developers to share their work with others who need specific versions or configurations set up on their side but lack administrator privileges – just provide them with your dockerfile along with documentation on how they should run it. This can save a lot of time and reduce the risk of errors or misconfigurations when setting up and deploying applications.

Overview of the article

Throughout this article, we will cover everything you need to know about Dockerfiles, from understanding their basics to advanced techniques for building customized images. First, we will examine the syntax and structure of Dockerfiles including common keywords like FROM, RUN, CMD, COPY and EXPOSE.

Next, we will guide you through the steps of building a simple Docker image with your own customizations. After that, we will dive into more advanced topics such as multi-stage builds for optimizing image size and performance as well as using ARG and ENV instructions to set environment variables.

We will also cover troubleshooting tips for debugging your Docker images built from scratch. By the end of this article, you’ll have a solid understanding of how to use Dockerfiles to create your own custom images that work seamlessly across different environments.

Understanding the Basics of Dockerfile

Syntax and Structure

In order to build a Docker image from scratch, one must first understand the syntax and structure of a Dockerfile. A Dockerfile is essentially a script that contains instructions for Docker to follow in order to create an image. The file consists of various commands, each starting with an instruction in capital letters followed by any arguments required for that instruction.

FROM Instruction

The FROM instruction is used to specify the base image that will be used for your new image. This can be any image available on the Docker Hub registry or a custom image you have previously created. For example, ‘FROM ubuntu:18.04’ would start with an Ubuntu 18.04 base image.

RUN Instruction

The RUN instruction allows you to execute commands within your container during the building process. This can be useful for installing packages or running scripts necessary for your application. For instance, ‘RUN apt-get update && apt-get install -y curl’ would update Ubuntu’s package list and install Curl.

CMD Instruction

The CMD instruction specifies what command should be run when your container is started from this newly created image. It is important to note that only one CMD instruction may be specified per Dockerfile, so it’s recommended that scripts are kept in separate files and run within this directive.

COPY Instruction

The COPY instruction allows you to copy files from your local machine into the container’s filesystem during build time. You can use this command to add configuration files or application code needed by your application.

EXPOSE Instruction

The EXPOSE instruction lets you specify which ports should be exposed by default when running a container built from this image without mapping them explicitly at runtime using ‘-p’, ‘-P’ options.

Best practices for writing a Dockerfile

To ensure your Dockerfile is easy to read, modify, and maintain, it’s recommended that you follow best practices. These include using a single RUN instruction to install multiple packages instead of splitting them into separate RUN commands, keeping the image size as small as possible by removing unnecessary files from the final image, using wildcards or variables to copy multiple files instead of listing each file individually in the COPY instruction.

Overall, understanding the basics of Dockerfile syntax and structure is essential to building an efficient and well-maintained Docker image from scratch. By adhering to best practices while using instructions like FROM, RUN, CMD, COPY and EXPOSE correctly you can streamline your development process while ensuring that your images are optimized in terms of both size and functionality.

Building a Simple Docker Image with a Dockerfile

Setting up the Environment

Before building a Docker image with a Dockerfile, it’s crucial to set up your environment properly. First, ensure that you have Docker installed on your machine by running the command “docker –version” in your terminal. If you don’t have it installed, you can download and install it from the official Docker website.

Next, create a new directory where you’ll store all the files related to building your image. This directory should contain the files needed by your application and the Dockerfile itself.

It’s best practice to keep each application isolated in its own folder with its own Dockerfile. Make sure that any tools or libraries required for building and running your application are installed in your environment.

Writing the Dockerfile Instructions

The next step is writing the instructions for building your Docker image. A basic template for a simple Node.js application might look like this:

FROM node:12-alpine

WORKDIR /app COPY package*.json ./

RUN npm install COPY . .

EXPOSE 3000 CMD [ "npm", "start" ]

This example starts with a base image of Node.js version 12 on an Alpine Linux distribution, sets up an application directory within the container at /app, copies over package.json and package-lock.json files to install dependencies using npm install, copies over all remaining files from our host machine into our container’s /app directory (this is where our code lives), exposes port 3000 so we can access it outside of our container and finally sets up our default CMD command to run when starting this container (“/usr/bin/npm start”). It’s important to note that each instruction in this example is cached separately by docker during build time allowing us faster rebuilds when only portions of our codebase have changed.

Building the Image

Once you’ve written your Dockerfile, it’s time to build your image. Navigate to the directory containing the Dockerfile in your terminal and run this command:

docker build -t imagename:tag .

This will tell Docker to build an image using the instructions in your Dockerfile and name it “imagename” with a specific version “tag”. The “.” at the end of the command specifies that we want docker to use the current directory as context for building our image.

The first time you run this command, it may take a while as Docker downloads all necessary dependencies and builds your application from scratch. However, subsequent builds will be much faster thanks to caching.

Building a simple Docker image from scratch using a Dockerfile can seem daunting at first, but once you understand how it works, it becomes easy and efficient. By following best practices like isolating each application within its own folder with its own unique Dockerfile and properly setting up environment variables, writing instructions for building images becomes less prone to errors when shared across teams.

Advanced Techniques for Building Customized Images with a Dockerfile

Multi-stage builds for optimizing image size and performance

One of the most significant challenges when building Docker images from scratch is keeping image size to a minimum. Docker images can quickly become large and unwieldy, making them difficult to work with and slowing down deployment times. One solution to this problem is multi-stage builds.

Multi-stage builds allow developers to split their Dockerfile into multiple stages, each with its own set of instructions. For instance, you might have one stage that installs dependencies and another stage that copies the application code into the image.

By separating these stages, you can optimize your images by only including what’s necessary in each layer. Another benefit of multi-stage builds is improved performance.

Because each stage is separate from the others, Docker only needs to rebuild those stages that have changed since the last build. This means faster build times and more efficient use of resources.

Using ARG and ENV instructions to set environment variables

Environment variables are an essential part of any application’s configuration. They provide a way to pass runtime information like database credentials or API keys into your container without hardcoding those values into your codebase.

Docker provides two primary ways to set environment variables in your container: ARG and ENV instructions in your Dockerfile. ARG allows you to pass arguments at build time that can be used later in your Dockerfile (e.g., as part of RUN commands).

On the other hand, ENV sets environment variables that are available when the container runs. Both ARG and ENV can be useful for customizing how your images are built or run based on external factors like development vs production environments or user-specific settings.

Working with volumes in a Docker container

Volumes are an essential feature of any production-level application running on containers. With volumes, you can store persistent data outside your container, ensuring that it’s available even when the container is stopped or restarted. Docker offers two types of volumes: host-mounted and anonymous.

Host-mounted volumes are created by specifying a path on the local host machine that will be mapped to a path inside the container. Anonymous volumes, on the other hand, are temporary and are created when a container is started with a specific flag.

Volumes can be set up in your Dockerfile using the VOLUME instruction. This allows you to map one or more directories from your container to the host machine at runtime.

Working with volumes in Docker can be complex, but it’s an essential skill for any developer building scalable and reliable applications. By carefully managing how your containers store and retrieve data, you can ensure that your applications run smoothly and efficiently.

Tips and Tricks for Debugging Your Docker Images Built from Scratch

Troubleshooting common errors when building images from scratch

Building Docker images can be a tricky process, especially when starting with a blank slate. It is common to encounter errors and issues during the build process.

Here are some common errors that you may encounter while building images from scratch and how to troubleshoot them:

1. “No such file or directory” error – This error is usually caused by an incorrect file path or name in the Dockerfile.

Double-check all file paths and make sure they are correct.

2. “Unable to locate package” error – This error occurs when you try to install a package that does not exist in the repository.

Make sure that the package name is correct and exists in the repository.

3. “Permission denied” error – This error occurs when you do not have sufficient permissions to access files or directories in your container.

Make sure that you have set appropriate permissions for your Dockerfile instructions.

4. “Connection refused” error – This error can occur if there is an issue with your network configuration inside the container, or if the service you are trying to connect to is not running properly.

Using docker build flags to debug your images

Docker provides several flags that can be used during the build process to help debug issues in your image:

1. –no-cache: This flag instructs Docker not to use any cached layers during the build process, forcing it to rebuild each layer from scratch.

2. –pull: This flag instructs Docker to always pull the latest version of a base image before starting the build process.

3. –progress: This flag controls how much progress information is displayed during the build process, allowing you to see more detailed output if needed.

4. –debug: This flag enables debugging mode during the build process, allowing you to see more detailed error messages and stack traces.

Creating custom logging in your images

Logging is an important part of any application, and Docker makes it easy to capture logs from your containers. However, sometimes you may need to create custom log messages to help with debugging or troubleshooting. Here are some ways that you can create custom logging in your images:

1. Use the logger command: The logger command allows you to write messages to the system log, which can be accessed using tools like syslog.

2. Use environment variables: You can use environment variables in your Dockerfile instructions to include custom log messages in your containers.

3. Use a logging driver: Docker provides several built-in logging drivers that allow you to capture logs from your container and send them to external systems like Elasticsearch or Graylog.

By following these tips and tricks for debugging your Docker images built from scratch, you can optimize the build process and ensure that your images are free from errors before deployment.


Throughout this article, we have discussed the essentials of Dockerfile, including its syntax, structure, and best practices for creating Docker images from scratch. We learned about the importance of each instruction and what role they play in building a Docker image. From the FROM instruction to the EXPOSE instruction, we explored how to write a Dockerfile that is optimized for performance and efficiency.

We covered advanced techniques such as multi-stage builds, using ARG and ENV instructions to set environment variables, and working with volumes in a Docker container. Additionally, we looked at tips and tricks for debugging images built from scratch through troubleshooting common errors when building images from scratch with docker build flags to debug your images.


Docker has revolutionized the way developers build and deploy applications by providing an efficient way to package code into containers. As more companies adopt containerization technology as part of their software development workflows, learning how to create custom images using Dockerfile will become increasingly important.

Looking ahead into the future, there will be more advancements in containerization technology that will make it even easier for developers to create custom images from scratch. Moreover, there will undoubtedly be new challenges that arise as more complex applications are developed.

Therefore developers must continue learning about new tools that can help them stay ahead of these challenges. Mastering Dockerfile is an essential skillset that all developers should learn.

Creating custom containers with optimized performance and efficiency is crucial for delivering applications quickly while maintaining high quality standards. With this knowledge at hand , you can take your application development skills to greater heights as you tackle new projects with confidence!

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