Docker is an open-source platform for developers to containerize applications and revolutionize the way they create, transport, and run applications. It has provided developers with lightweight and self-sufficient containers that contain everything from code and runtime to libraries and dependencies to run applications, irrespective of their deployment source. This innovative approach eradicates the prevalent issue of "it doesn’t work on my machine," enhancing the dependability and smoothness of both development and deployment processes.
At the core of Docker's capabilities lies a straightforward yet potent text file known as "Dockerfile," which streamlines the creation of Docker images. Whether you are developing an application, testing it in different environments, or deploying it at a scale, understanding the basics of Dockerfiles is crucial. In this guide, we will shed light on what Dockerfiles are, their instructions, creation, benefits, and how it streamlines the Docker image-building process. Mastering Dockerfiles, whether you're a developer, system administrator, or DevOps engineer, can drastically improve your workflow and productivity.
Docker Dictionary: Understanding Key Terms
Before delving into the details of Dockerfiles, it's crucial to understand what exactly docker and its components are. Have a look at the basic docker terminologies to understand the platform better:
Docker: It is a software platform for developing, shipping, and running applications. It allows you to package applications and dependencies into a single unit, enabling quick software delivery.
Container: It is a standardized unit of software that contains code and all of its dependencies, allowing the application to execute rapidly and consistently across multiple computing environments.
Docker Image: It is a layered file system containing the source code, dependencies, libraries, tools, and other files and instructions needed for creating a Docker container.
Dockerfile: It is a text file that contains a series of instructions called Docker commands to build a Docker image.
Docker Build Context: It is a Docker host location that contains all of the codes, configurations, files, and Dockerfiles created throughout the building process.
What is a Dockerfile?
Dockerfile is a powerful text file that contains a blueprint for generating customized Docker images. The instructions and steps present in a Dockerfile, like installing dependencies, setting variables, copying files, configuring the container, and explaining how to automate and build docker images. Each instruction adds a new layer to the image when you run the "docker build" command in the directory. The final image is the result of all the layers stacked on top of one another. The generated image can then be deployed as a container with the “Docker run” command. The Dockerfile allows you to convert your apps into portable containers that can run anywhere. Besides that, Dockerfile commands have a wide range of purposes like:
- Set the Base Image
- Install application dependencies.
- Execute commands
- Define runtime configuration.
- Specify the container environment variables.
- Copy application files and directories
- Set up application directories.
- Define default commands
- Provide image metadata.
Main Instructions of a Dockerfile
The first thing you need to understand about Dockerfile is its commands. It contains a set of instructions that help you in the creation of the Dockerfile as well as Docker image. Here are some frequently used instructions:
FROM: defines the foundational image utilized to initiate the build process.
RUN: central executing directive that executes a command in the container.
ADD: copies the files/ directories from a source on the host into the container.
WORKDIR: specifies the path where the command defined by CMD will be executed.
CMD: Specifies the default command that executes when the container starts.
ENTRYPOINT: Set up a container to execute automatically whenever an instance is created using the image.
ENV: sets environment variables.
EXPOSE: specifies the particular ports to make the container listen and network with the outside world.
MAINTAINER: sets the author field to indicate who created and maintains the Docker image by determining the name and email address of the creator.
USER: sets the user and, optionally, the user group for running commands within the Docker container.
VOLUME: enables access from the container to a directory on the host computer.
LABEL: adds metadata to an image, such as information about the image, including version, description, etc.
How to create a Dockerfile from Scratch?
The first step in using the Dockerfile is to create it properly. It involves understanding key components and important steps to follow to define the environment and behavior of the container. Take a look at the step-by-step guide to creating your first Dockerfile:
- Open IDE and navigate to the project directory.
- Use your preferred text editor to create a Dockerfile.
- Every Dockerfile starts with a base image. Therefore, specify the base image using the FROM instruction (FROM <image>). For example, FROM ubuntu:latest.
- Use WORKDIR instruction to set the working directory inside the container.
- Use COPY instruction to copy files and directories from your host machine to the current directory.
- Use the RUN instruction to execute commands in the container that will run during image building, such as installing dependencies.
- Set up the CMD instruction, which specifies the command that will be executed when the container starts.
- Use ENV instruction to set environment variables within the image/container.
- Use the EXPOSE command to notify Docker which ports the container will listen on during execution.
How to Build Docker Image Using Dockerfile?
A standard Docker image comprises a base image layer and additional image layers to create a complete image and modify the base one. Dockerfiles simplify the image-building process using various instructions. Here is a step-by-step process guide on building a Docker image using a Dockerfile:
Step 1: Create a Working Directory
Create a directory to store your project files, including the Dockerfile using:
mkdir <directory>
For example - mkdirmyproject
Step 2: Create an HTML File
Create a file named index.html and add the following content:
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Simple App</title>
</head>
<body>
<h1>Hello Everyone</h1>
<p>This is running in a docker container</p>
</body>
</html>
Step 3 - Select a Base image
After that, select a suitable base image like nginx, ubuntu,etc., from a local repository or Docker Hub to serve as the foundation of your custom image. The base image serves as the foundation for your Docker image, encompassing the operating system and essential dependencies.
Step 4 - Create a Dockerfile
Now, create a file called "Dockerfile." This file contains the build instructions for your image. When you execute the docker build command, it looks for the Dockerfile.
Step 5 - Build the Docker Image
Make sure Docker is installed and running on your machine using the docker --version command.
To build a Docker image, execute the following command, substituting your desired image name and path accordingly:
docker build -t <image_name> <path>
Next, execute the following command to build the Docker image:
Docker build -t <image_name> .
Example: docker build -t myimage .
Make sure the Dockerfile is present in the current directory denoted by ( . ).
Step 6: Verify the Built Docker Image
List the Docker images to ensure that your image was successfully built with the "docker images" command, and then display it in the list of local images.
Step 7: Run the Docker Image
Run a container from the built image:
docker run --name <container_name> <image_name>
For Example: docker run --name mycontainermyimage
Best Practices for creating and optimizing Dockerfiles
- Keep Dockerfiles Lean: Ensure to keep the Dockerfiles concise and organized by minimizing the number of layers by combining commands when possible. Every RUN, COPY, and ADD command creates a new layer, so it's best to combine commands when possible to optimize efficiency.
- Use Lightweight Images: Use lightweight images as the base image to start building docker images. Smaller images improve performance as well as reduce resource consumption during deployment and minimize attack surfaces.
- Label Images: There are different images and it's better to label them for better organization using LABEL instruction.
- Use .dockerignore: Use this to exclude unnecessary files and directories from the build context and ensure you're adding the required files only.
- Leverage Caching: Accelerate subsequent builds using Docker's caching technique. Use layers that do not change regularly or use the --no-cache=true flag if needed. To maximize the use of docker's layer caching, order instructions from least to most frequently change.
- Use Multi-Stage Builds: For complex builds, use multi-stage builds to separate build dependencies from the final runtime image and keep the final image size small.
- Non-Root User: Consider running as a non-root user to avoid security breaches and enhance container isolation.
- Health check: Use HEALTHCHECK to check the container's health and get info on when it enters a failed state.
- Avoid Multiple RUN commands: Use fewer RUN commands to avoid creating multiple cacheable layers, which can affect the efficiency of the image build process.
- Use a Linter: Check your Dockerfile for common problems, missing instructions, syntax errors, and security best practices using a linter tool such as hadolint.
- Use a Single Process per Container: Use just one process per container to keep containers lighter and facilitate their management and monitoring.
Conclusion
Dockerfiles are powerful tools for automating the creation of Docker images. We have discussed everything about Dockerfiles, from its meaning and instructions to creation and best practices. Dockerfiles contain a set of instructions that can help you build docker images efficiently and successfully. They simplify development and deployment processes, making applications more portable and manageable. Each of its instructions serves various purposes that make Dockerfiles and docker image creation process streamlined and seamless. Understanding and implementing the basic instructions and best practices will allow you to efficiently build, manage, and deploy your applications in an enclosed environment.
