build: devcontainer update

[janfb]

fixes #1251 by adding a dispatch workflow to the docker GH action.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.16%. Comparing base (6dec812) to head (386f1ee).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1252      +/-   ##
==========================================
- Coverage   86.05%   78.16%   -7.90%     
==========================================
  Files         118      118              
  Lines        8672     8679       +7     
==========================================
- Hits         7463     6784     -679     
- Misses       1209     1895     +686     

Flag	Coverage Δ
unittests	78.16% <ø> (-7.90%)	⬇️

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see 28 files with indirect coverage changes