docs: Note how to use the Docker image from within GitLab CI

[eddybl]

Ref: #2823

Changes

Improves the main README to give an example as to how to use the Docker image within GitLab CI..

Discussion: #2822
Issue: #2823

Documentation and testing

Please consider whether this PR needs documentation and tests. This is not required, but highly appreciated:

• Documentation in the matching docs section
• Unit tests and/or functional tests

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.50%. Comparing base (7ec3189) to head (6b9b069).
Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2824      +/-   ##
==========================================
- Coverage   96.52%   96.50%   -0.02%     
==========================================
  Files          90       90              
  Lines        5872     5873       +1     
==========================================
  Hits         5668     5668              
- Misses        204      205       +1     

Flag	Coverage Δ
api_func_v4	82.22% <ø> (-0.02%)	⬇️
cli_func_v4	83.63% <ø> (+0.05%)	⬆️
unit	88.28% <ø> (-0.02%)	⬇️

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[nejch]

Thanks a lot for the docs @eddybl! LGTM, maybe we can sync the version with our tags later so we don't recommend using latest to avoid breaking changes. But not in the scope of this PR maybe.