Dockerfile

Abstract

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• Set of steps for Docker to build Docker Container

Important

We can use a .dockerignore file to exclude files from being copied into a Docker image. It’s best practice to copy over the dependency files and install dependencies before copying the application files to take advantage of layer caching.

Keyword at Runtime

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• Keywords that have an effect when Docker Container is running

ENV

• Set the Environment Variable in the Docker Container

CMD

• Purpose: Provides default commands or arguments that can be overridden at runtime. If you specify a command when starting the container (docker run <image> <command> <args>), it overrides the CMD
• Typically used for setting default arguments when a container starts (e.g., parameters for the main application specified with ENTRYPOINT)
• Exec form: CMD ["executable", "param1", "param2"] (preferred as it doesn’t run in a shell). Example CMD ["python", "app.py"]

ENTRYPOINT

• Purpose: Defines the main command that will always run in the container, and you can optionally provide default arguments with CMD. It is not overridden by arguments passed to docker run <image> <args> (unlike CMD). The container will always execute the command defined in ENTRYPOINT, and any arguments passed at runtime will be appended to it
• Exec form: ENTRYPOINT ["executable", "param1"] (recommended)

CMD + ENTRYPOINT

ENTRYPOINT ["python"] 
CMD ["app.py"]

• ENTRYPOINT specifies the command that will always run
• CMD provides default arguments to the ENTRYPOINT
• In the above example, if you run docker run <image> script.py, it will execute python script.py because CMD can be overridden
• CMD does not execute as a separate process when ENTRYPOINT specifies a complete command. Instead, it is effectively ignored in that context.
• Refer to HashiCorp Vault Dockerfile for an example