Update ArgoCD example Dockerfile in documentation

[TheGthr]

What this PR does / why we need it: The Dockerfile in the documentation mentions the old ArgoCD repository on Docker Hub. The new one, with latest updated images, is located on quay.io. I updated the example Dockerfile for the developers to be able to copy its content and make it work as is.

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged):

Special notes for your reviewer: I also added Sopra Steria in the USERS.md file, should I do a separate PR?

PR Readiness Checklist:

Complete these before marking the PR as ready to review:

• no need for the CHANGELOG.md to be updated as it is only a documentation fix

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 87.41%. Comparing base (3ce0af7) to head (5b44c7a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #493      +/-   ##
==========================================
- Coverage   87.42%   87.41%   -0.02%     
==========================================
  Files          22       22              
  Lines         843      842       -1     
==========================================
- Hits          737      736       -1     
  Misses        106      106              

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[jkroepke]

Thanks a lot! I'm happy about the note on the USERS.md