Dockerfile: factor out Coccinelle version into arg, reformat

• apt-get shall be used for automated CLI (see https://manpages.ubuntu.com/manpages/focal/en/man8/apt.8.html#script%20usage%20and%20differences%20from%20other%20apt%20tools)
• autoremove will clean up automatically installed dependencies that are no longer needed, and --purge will remove their traces