[MM-53555] Implement env override for job service image registry

[streamer45]

Summary

We introduce a new MM_CALLS_JOB_SERVICE_IMAGE_REGISTRY override to control the image registry to be used to define job runners. This is needed in case something other than the default public registry (mattermost) is configured on the calls-offloader side (see related PR).

Related PR

mattermost/calls-offloader#50

Ticket Link

https://mattermost.atlassian.net/browse/MM-53555

[codecov-commenter]

Codecov Report

Attention: 7 lines in your changes are missing coverage. Please review.

Comparison is base (d7caacc) 9.33% compared to head (a93631b) 9.32%.

Files	Patch %	Lines
server/job_service.go	0.00%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main    #607      +/-   ##
========================================
- Coverage   9.33%   9.32%   -0.01%     
========================================
  Files         26      26              
  Lines       5294    5299       +5     
========================================
  Hits         494     494              
- Misses      4748    4753       +5     
  Partials      52      52              

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