Use registry cache for Docker workflows

[FloWuenne]

Summary

• Replace Docker Buildx GitHub Actions cache backend with GHCR registry cache refs in PR and release image workflows.
• Keep workflow action uses: references pinned to full commit SHAs.

Testing

• Passed focused workflow validation confirming all workflow uses: refs are full 40-character SHAs.
• Passed focused scan confirming no actions/cache@, type=gha, or tag-pinned uses: ...@vN references remain.
• Passed YAML syntax parsing for all workflow files.
• actionlint is not installed in this environment; no native actionlint pass was available.
• GitHub reported no PR checks yet for this branch at validation time.