ci: Make the container image registry configurable.

[jackluo923]

Description

Makes the container image registry host and credentials configurable via a workflow-level env block backed by repo variable REGISTRY and secrets REGISTRY_USERNAME / REGISTRY_PASSWORD, defaulting to ghcr.io + ${{github.actor}} + ${{secrets.GITHUB_TOKEN}}. clp-core-build-containers and run-on-image gain image_registry* inputs mirroring clp-build-runtime-image. run-on-image only logs in when a registry username is configured, so OSS keeps pulling published images anonymously from public ghcr.io — behavior is unchanged when the variable and secrets are unset.

Checklist

• The PR satisfies the contribution guidelines.
• This isn't a Breaking change.
• No docs need to be updated.

Validation performed

• yamllint clean on the three changed files; actionlint reports no new findings (only the pre-existing SC2046 chown warnings repo-wide).
• All CI checks on the branch pass (to be confirmed once the run completes).

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