feat(CI): add cosign verification for Docker images

[pellared]

Towards #1201

As cosign verify was not working for me some time ago I prefer to add signature verification to the CI.

It is also recomended in https://docs.sigstore.dev/quickstart/quickstart-ci/#signing-and-verifying-a-container-image

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.35%. Comparing base (ce8adf4) to head (601f835).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1333       +/-   ##
===========================================
+ Coverage   19.60%   44.35%   +24.75%     
===========================================
  Files         242      306       +64     
  Lines       28070    27760      -310     
===========================================
+ Hits         5502    12314     +6812     
+ Misses      21922    14713     -7209     
- Partials      646      733       +87     

Flag	Coverage Δ
integration-test	?
integration-test-arm	?
integration-test-vm-x86_64-5.15.152	?
integration-test-vm-x86_64-6.10.6	?
k8s-integration-test	?
oats-test	?
unittests	44.35% <ø> (?)

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[grcevski]

LGTM!