[DOP-14540] Allow Docker entrypoint to automatically create SUPERADMIN users

[dolfinus]

Change Summary

Now Docker image entrypoint can automatically create SUPERADMIN users during startup, using HORIZON__ENTRYPOINT__ADMIN_USERS env variable.

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.68%. Comparing base (c94d748) to head (3d3608d).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #45   +/-   ##
========================================
  Coverage    90.68%   90.68%           
========================================
  Files          120      120           
  Lines         2416     2416           
  Branches       231      231           
========================================
  Hits          2191     2191           
  Misses         174      174           
  Partials        51       51           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.