Fix docker build on BuildKit (now the default on Actions runners)

[ian-h-chamberlain]

This may be easiest to review commit-by-commit, particularly the Dockerfile. I tested the build on my fork with my own credentials to make sure everything was working as well.

Changes:

• Runners use Docker v24 now, which uses BuildKit by default. This broke the build when reinstalling filesystem due to BuildKit builds (via Docker) are broken if /etc/hosts or /etc/resolv.conf is replaced moby/buildkit#1267
• Expand out the package list in the Dockerfile for easier diffs when adding/removing packages.
• Add mesa-unstable to the list of removed packages, it had some dependencies on other packages that are being removed
• Extract version info from the built image, and add tags for the version of SteamOS as well based on discussion from Latest image can't sync package database #4 (comment)

[AAGaming00]

see comments, not directly approving but not explicitly requiring the version detection change

[ian-h-chamberlain]

Hey @AAGaming00, it seems like Valve updated the API to actually contain image versions now, so I went ahead and implemented that change, and rebased onto the latest main.

Since 3.5.7 is out on stable now I suppose that building that might be a good test of the changes?