Add workflow to lint Dockerfile using dockerfilelint

[make-github-pseudonymous-again]

This fixes #969. Since dockerfilelint is not really maintained and hadolint is a much better option (but cannot be used right now), we run it using npx on GHA directly without adding it as a devDependency to package.json.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.86%. Comparing base (6308ea8) to head (e5533de).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #974      +/-   ##
==========================================
- Coverage   65.88%   65.86%   -0.03%     
==========================================
  Files         715      715              
  Lines        9893     9893              
  Branches     1346     1346              
==========================================
- Hits         6518     6516       -2     
- Misses       2917     2918       +1     
- Partials      458      459       +1     

Flag	Coverage Δ
client	46.23% <ø> (-0.03%)	⬇️
server	61.87% <ø> (ø)
test	71.97% <ø> (ø)
test-app	53.70% <ø> (-0.03%)	⬇️

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