Add allowed_sites restriction to Docker images

[MrCreosote]

Lets admins restrict an image to specific compute sites at registration time; job submission is rejected with IllegalParameterError if the target cluster isn't in the image's allowed_sites.

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.23%. Comparing base (8980d08) to head (547516c).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
cdmtaskservice/routes.py	16.66%	10 Missing ⚠️
cdmtaskservice/models.py	44.44%	5 Missing ⚠️
cdmtaskservice/images.py	40.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #736      +/-   ##
==========================================
- Coverage   64.33%   64.23%   -0.11%     
==========================================
  Files          68       68              
  Lines        6878     6906      +28     
==========================================
+ Hits         4425     4436      +11     
- Misses       2453     2470      +17     

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