fix: ability to pass imageTagMutability via config

[estahn]

fixes #649

[codecov]

Codecov Report

Attention: Patch coverage is 23.52941% with 26 lines in your changes are missing coverage. Please review.

Project coverage is 34.77%. Comparing base (910dafc) to head (f856c8b).
Report is 7 commits behind head on main.

Files	Patch %	Lines
pkg/registry/ecr.go	13.33%	26 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #678      +/-   ##
==========================================
+ Coverage   34.59%   34.77%   +0.18%     
==========================================
  Files           9        9              
  Lines         688      693       +5     
==========================================
+ Hits          238      241       +3     
- Misses        425      427       +2     
  Partials       25       25              

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