Align build steps - distinguish clean vs dev builds

[jotak]

• Reducing the Dockerfile dependencies allows faster dev builds
• Rename target build_code -> compile for consistency
• Remove now useless validate_go target
• Remove confgenerator from docker images (we may introduce another dockerfile if someone finds it useful and want it back)

[openshift-ci]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.38%. Comparing base (753c34a) to head (8882e0f).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #745      +/-   ##
==========================================
- Coverage   64.39%   64.38%   -0.01%     
==========================================
  Files         108      108              
  Lines        8303     8301       -2     
==========================================
- Hits         5347     5345       -2     
  Misses       2638     2638              
  Partials      318      318              

Flag	Coverage Δ
unittests	64.38% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
cmd/confgenerator/main.go	61.79% <100.00%> (-0.43%)	⬇️
cmd/flowlogs-pipeline/main.go	54.33% <100.00%> (-0.36%)	⬇️