What does `docker build` do?

Great question!

In simple terms, docker build takes your Dockerfile (the "blueprint" you just created) and turns it into a Docker Image (the actual "package" or "template").

Here is a breakdown of what happens when you run sudo docker build -t ssh-server .:

1. Read the Instructions: Docker reads the Dockerfile in your current directory (that's what the . at the end of the command stands for).
2. Execute the Steps: It goes through the file line by line:
   • It starts with the base OS (FROM ubuntu:22.04).
   • It runs the commands to install software (RUN apt-get update...).
   • It sets up the user and configurations you defined.
3. Create Layers: Each instruction creates a "layer" in the image. This makes it efficient because if you change one line later, Docker only has to rebuild that specific part.
4. Tag the Result: The -t ssh-server part gives your finished image a name ("ssh-server"), making it easy to reference later.

Analogy:

• Dockerfile: A recipe.
• docker build: The act of cooking.
• Docker Image: The finished cake (which you can then "serve" or run as a container).

Once the build is finished, the image is stored on your machine, ready to be launched as a running container!