Enhance Dockerfile instructions and explanations

[mruiserrmendix]

Added explanations for Dockerfile commands and clarified the image build process.

[katarzyna-koltun-mx]

@mruiserrmendix all of the changes you suggested in the PR are already included in the docs, in the form of script comments:

   # This file provides an example on how to start the runtime in Docker.
   # It is based on the configuration named Default.

   # Start from an JAVA base image, as the Portable Package contains all necessary dependencies
   FROM eclipse-temurin:21-jdk

   # Set the working directory to /app
   WORKDIR /mendix

   # Copy the contents of the Portable Package to the /app directory in the image
   COPY ./app ./app
   COPY ./bin ./bin
   COPY ./etc ./etc
   COPY ./lib ./lib

   # Set environment variables (optional)
   ENV MX_LOG_LEVEL=info
   ENV M2EE_ADMIN_PASS=${M2EE_ADMIN_PASS}

   # Expose port 8080 for the Mendix Runtime and port 8090 for the Mendix Runtime admin interface
   EXPOSE 8090
   EXPOSE 8080

   # Set the start script to the Mendix Runtime execute command
   CMD ["./bin/start", "etc/Default"]
   ```

Because of that, I'm going to close this PR. Let me know if you'd like to discuss this.

[mruiserrmendix]

My bad, you are right. I didn't have my daily quantity of coffee that day. :D