fix: Ensure container users always exist

[dionhaefner]

Relevant issue or PR

Fixes #425

Description of changes

• Add addmeplease executable to every container that adds the current user + group to /etc/passwd and a few other system files.
• This ensures user code can now rely on the fact that the executing OS user always has a proper uid, gid, home directory, and shell.

We achieve this by compiling a simple C binary that we can run as root with setuid even from non-privileged accounts.

Testing done

Tested on reproducer from #425, and added new e2e test on CI.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 76.76%. Comparing base (5284bb6) to head (90823bb).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #427      +/-   ##
==========================================
+ Coverage   66.91%   76.76%   +9.84%     
==========================================
  Files          29       29              
  Lines        3431     3434       +3     
  Branches      535      536       +1     
==========================================
+ Hits         2296     2636     +340     
+ Misses        945      569     -376     
- Partials      190      229      +39     

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[apaleyes]

delete all commented stuff?

[apaleyes]

and here?

[apaleyes]

and this file?

[apaleyes]

Somewhere, maybe here in the description, it would be nice to have an explanation of what this example demonstrates

[apaleyes]

After thinking about this for a while and reading docker/linux docs, I am happy to approve it. Still can't claim full understanding though! But at least the general idea now makes sense.

Of course it would be nice to have @xalelax see this too, as he was involved in all earlier bugs/complications with setting users inside tesseracts