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 76.81%. Comparing base (21b7351) to head (f5cc8e6).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #427      +/-   ##
==========================================
+ Coverage   76.70%   76.81%   +0.10%     
==========================================
  Files          29       29              
  Lines        3439     3442       +3     
  Branches      537      538       +1     
==========================================
+ Hits         2638     2644       +6     
+ Misses        569      567       -2     
+ Partials      232      231       -1     

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[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

[xalelax]

I was originally puzzled by the approach, but tbh while looking for alternatives I didn't find anything better. Let's go with this

[xalelax]

I went ahead and implemented @apaleyes 's suggestions on 8092b02; merging soon