What is Dockerfile and How to Create a Dockerfile in 2025?

What is Docker?

Docker is a configuration management tool for automating software deployment in lightweight containers. These containers help applications work efficiently in different environments.

Features of Docker:

• Easy and faster configuration
• Application isolation
• Security management
• High productivity
• High scalability
• Infrastructure independent

A Docker container is a software package with all the dependencies required to run an application.

Note: A Docker Image is a template of instructions used to create containers

What is a Dockerfile?

Before we discuss what is a Dockerfile, it is essential to know what a Docker image is.

Docker Image

A Docker Image is a read-only file with instructions that create a Docker container when executed.

Dockerfile

Dockerfile is a simple text file with instructions to build Docker images.

Mentioned below is the syntax of a Dockerfile:

Syntax

# comments

command argument argument1...

Example

# Print "Get Certified. Get Ahead"

Run echo "Get Certified. Get Ahead"

Now, let's look at how to build a Docker image using a dockerfile.

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Build a Docker Image with Dockerfile

Let’s first declare the path where we will be storing the dockerfile simplidocker

docker build [OPTIONS] PATH | URL | -

Now, let’s build a basic image using a Dockerfile: 

docker build [location of your dockerfile]

Now, by adding -t flag, the new image can be tagged with a name:

docker build -t simpli_image

Once the Docker image is created, you can verify by executing the command:

docker images

The output should show simpli_docker, which is available in the repository.

Create a New Container

Now, create a Docker container from the Docker image we created in the previous step.

Let’s name the container “simplilearn” and create it with the command:

docker run --name simplilearn simpli_docker

The message ‘Welcome to Simplilearn’ should appear in the command line, as seen in the image above.

Congratulations, you just created a Docker image and a Docker container using a Dockerfile.

Benefits of Dockerfile

Here are the potential benefits of Dockerfile:

1. Automated Builds: Dockerfile automates creating Docker images, ensuring consistent and reproducible builds every time.
2. Version Control: Dockerfiles can be versioned just like source code, making it easier to track changes, revert to previous versions, and maintain different configurations.
3. Environment Consistency: Using a Dockerfile ensures the application runs in the same environment across development, testing, and production, reducing the "it works on my machine" problem.
4. Layered File System: Dockerfiles build images in layers, allowing for caching of layers and faster builds. Changes only affect the layers that need rebuilding, optimizing the build process.
5. Ease of Sharing: Dockerfiles can be shared and reused, making it simple to distribute applications and their dependencies across different teams and environments.
6. Cross-Platform Compatibility: With Docker, applications can run consistently on any platform that supports Docker, promoting portability and flexibility.

Best Practices for writing Dockefile

• Use Official Base Images: Start with official base images from Docker Hub. They are optimized and maintained, ensuring better security and performance.
• Minimize Layers: Combine commands into single RUN statements to reduce the number of layers in the final image. This minimizes image size and improves build time.
• Order Instructions Wisely: Place frequently changing instructions (like COPY or ADD) at the end of the Dockerfile. This allows Docker to cache earlier layers and speed up subsequent builds.
• Use .dockerignore: Create a .dockerignore file to exclude unnecessary files and directories from being included in the build context, reducing image size and build time.
• Specify Exact Versions: Always specify exact versions of dependencies in your Dockerfile to prevent unexpected changes when rebuilding the image.
• Clean Up: After installation, remove unnecessary packages and files to keep the image lightweight. Use apt-get clean or similar commands to achieve this.
• Use Multi-Stage Builds: For larger applications, multi-stage builds reduce the size of the final image by separating the build environment from the runtime environment.

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Troubleshooting of Dockerfile Issues

You can follow the below steps to troubleshoot Dockerfile issues.

1. Check Build Logs: When encountering issues, carefully examine the output of the docker build command for error messages or warnings that indicate where the problem occurred.
2. Run Docker Commands in Interactive Mode: Use the docker run -it command to start a container based on your image and troubleshoot interactively. This allows you to explore the file system and test commands.
3. Use the Dockerfile Linter: Tools like Hadolint can analyze your Dockerfile for common mistakes and suggest improvements.
4. Check Network Issues: If your Dockerfile requires downloading dependencies, ensure your network settings are correctly configured, and the base images or package repositories are accessible.
5. Debug with Intermediate Images: If a build fails, use intermediate images (with specific steps in your Dockerfile) to isolate the problem. You can tag and run these intermediate images for further testing.
6. Ensure Correct File Paths: Verify that file paths specified in COPY and ADD commands are correct and that the files exist in the build context.
7. Review Permissions: Ensure you have the necessary permissions for files and directories accessed or modified in the Dockerfile. Failures during the build process can result from this.

Conclusion

With that, we have come to the end of the article on what a docker file is. In this write-up, we discussed what a docker is, the syntax of a docker file, important commands to create a docker image using a docker file, and how to build a docker image using a docker file. 

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FAQs

1. When should I use Dockerfile?

One should use a Dockerfile when there’s a need to distribute/collaborate on the app’s operating system with a team. Use Dockerfile as the version control system for the entire app’s OS. Or use Dockerfile to run the code on the laptop in the same environment as the server you are working on. 

2. Is Dockerfile a text file?

Dockerfile is a text document containing all the commands the user requires to call on the command line to assemble an image. With the help of a Dockerfile, users can create an automated build that executes several command-line instructions in succession. 

3. How do I create a simple Dockerfile?

To create a Dockerfile, set up Docker and Docker Hub. Create the original Docker container and then create a file on it. Make changes to the container, and finally, create a new image. 

4. What is Dockerfile language?

The Go language is used to write Docker. A Dockerfile is a text file that contains collections of instructions and commands that will be automatically executed in sequence in the Docker environment to build a new Docker image. 

5. What is Docker compose vs. Dockerfile?

The key difference between Docker Composer and Docker is that the Docker contents describe how to create and build a Docker image. In contrast, Docker compose is a command that runs Docker containers based on settings described in a docker-composed.yaml file. 

6. What is a .dockerignore file?

A .dockerignore file allows you to specify a list of files or directories that Docker will ignore during the build process. It is similar to a .gitignore file used when you build Git repositories. You can specify the list of files and directories inside the .dockerignore file. 

7. How do I commit a docker container?

First, you need to pull a docker image. Then, the container is deployed and modified, and changes are committed to the image. When you commit to changes, you create a new image with an additional layer that modifies the base image layer. 

8. What is docker context?

With a Docker context, importing and exporting context on different machines with the Docker installed becomes easier. One can also use a Docker context export command to export an existing context to a file, which can be imported to another machine with the Docker client installed. 

9. What is the use of Docker in DevOps?

A Docker container image in DevOps is a lightweight, executable, and standalone package of software that includes everything needed to run an app, runtime, system tools, settings, system libraries, and code. Containers simplify the build/test/deploy pipelines in DevOps.  

10. Is Dockerfile COPY recursive?

COPY and ADD are the two commands that Docker provides for copying files from the host to the Docker image when building it. COPY command copies files recursively, given explicit source and destination directories or files.

11. Is Dockerfile a Docker image?

A Docker image is built automatically by reading the instructions from a Dockerfile, a text file containing all commands needed to build a given image. A Dockerfile adheres to a specific set of instructions and format, producing a Docker image when you build it.

12. What does Dockerfile RUN do?

The RUN instruction in a Dockerfile executes commands inside the container during the image build process. It allows you to install software, configure the environment, or perform tasks like downloading dependencies. Each RUN command creates a new layer in the Docker image.

13. What is the difference between Docker and Dockerfile?

Docker is a platform that allows you to create, deploy, and manage containers. A Dockerfile, on the other hand, is a text file that contains a series of instructions for building a Docker image. Dockerfile defines the steps for creating an image, while Docker executes and manages the images.

14. What does the Dockerfile build do?

The docker build command creates a Docker image using the instructions in a Dockerfile. It processes each instruction (like COPY, RUN, CMD) step by step to generate the final image, which can then be used to create containers.

15. What is a Dockerfile extension?

A Dockerfile doesn’t have a specific file extension by default. It is typically named Dockerfile without any extension. However, you can name it with custom extensions (like Dockerfile.dev) if you want to specify different configurations for different environments.

16. Why is CMD used in Dockerfile?

The CMD instruction in a Dockerfile specifies the default command to execute when a container starts. It defines the command and arguments to run within the container, but it can be overridden by specifying a different command during the docker run.