Fix Docker Registry Authentication in GitHub Actions

[tekenstam]

What this PR does / why we need it

This PR fixes the image-push workflow that was failing with the error "No credentials found for registry keikoproj". The issue was caused by not explicitly specifying the Docker registry, causing Docker to interpret "keikoproj" as a registry name rather than an organization on DockerHub.

Changes made

• Added explicit docker.io/ prefix to all Docker image references in the workflow
• Updated build outputs configuration to use the fully qualified image name
• Updated attestation subject names to use consistent registry references

How to verify it

The image-push GitHub Action workflow should now successfully authenticate and push images to both DockerHub and GitHub Container Registry.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 46.13%. Comparing base (8dd20b1) to head (1d60a0b).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #477   +/-   ##
=======================================
  Coverage   46.13%   46.13%           
=======================================
  Files          40       40           
  Lines        7004     7004           
=======================================
  Hits         3231     3231           
  Misses       3617     3617           
  Partials      156      156           

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