ci: Nightly mainline docker builds

[another-rex]

We only enabled osv-scanner-action nightly docker builds, but we should do the same for the main dockerfile as well.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.94%. Comparing base (ff8ddf0) to head (a360036).
⚠️ Report is 320 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2298      +/-   ##
==========================================
+ Coverage   67.03%   67.94%   +0.91%     
==========================================
  Files         171      171              
  Lines       16403    12785    -3618     
==========================================
- Hits        10996     8687    -2309     
+ Misses       4734     3425    -1309     
  Partials      673      673              

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