2024-12-31 Dockerfile Explained

Dockerfile Explained

What is a dockerfile?

• Step by step instructions to build Image
• Blueprint of a container
• Written in go
• Has Instructions

Some Basic Instructions are:

Instructions	Use Case
FROM	Specifies the base image to start with. example: nginx or ubuntu
COPY	Copies files or directories from your computer into the image.
RUN	Executes commands within the image during the build process
ADD	Similar to COPY, but can also extract compressed files and download from URLs.
EXPOSE	Informs Docker that the container listens on a specific network port at runtime.
CMD	Specifies the command to run when the container starts.
ENTRYPOINT	Main command that always executes when the container starts.

There are two ways to run command in container.

1. Using CMD instruction
2. Using Entrypoint

CMD

• Purpose: Defines the default command to execute when the container starts.
• Flexibility: Easily overridden by using the docker run command with a different command.
• Example: CMD ["python", "app.py"]

ENTRYPOINT

• Purpose: Configures the container to run as an executable. It sets the main command that always executes.
• Less Flexible: While arguments can be appended, the core command defined by ENTRYPOINT remains.
• Example: ENTRYPOINT ["/usr/bin/nginx"]

Key Differences and Use Cases

Feature	CMD	ENTRYPOINT
Override Behavior	Easily overridden by docker run	Not easily overridden; arguments can be appended
Execution	Runs after ENTRYPOINT if both are defined	Runs first, effectively becoming the container’s primary executable
Typical Use Cases	Providing a default command, which can be changed if needed	Setting a fixed executable for the container, like a web server

Entrypoint file can be single command or shell script. Simple example of entrypoint script is this: Entrypoint.sh

#!/bin/bash
 
# Check if a configuration file exists
if [ ! -f /app/config.yml ]; then
  echo "Error: config.yml not found!"
  exit 1
fi
 
# Start the application
exec python /app/app.py

In your Dockerfile, you would use:

Dockerfile

COPY entrypoint.sh /entrypoint.sh
RUN chmod +x /entrypoint.sh
ENTRYPOINT ["/entrypoint.sh"]

Example of Dockerfile using command for entrypoint Dockerfile

FROM ubuntu:latest
 
# Install Apache
RUN apt-get update && apt-get install -y apache2
 
# Set the entrypoint to the Apache server
ENTRYPOINT ["apache2ctl"]
 
# Default command to start Apache in the foreground
CMD ["-D", "FOREGROUND"] 

In this example, ENTRYPOINT ["apache2ctl"] configures the container to run the Apache web server as its main process. When you start a container from this image, it will always execute apache2ctl.

---

Homework

• Follow on Task 5 Create dockerfiles