Configure Dependabot updates for pinned Docker image digests

[brooke-hamilton]

Area for Improvement

Radius pins runtime base images by digest for reproducible builds, but the repository's generated Dependabot configuration does not enable the docker ecosystem.

Observed behavior

Pinned Docker image digests must be discovered and refreshed manually. .github/dependabot.yml is synced from radius-project/.github, so a local-only edit in this repository would be overwritten.

Desired behavior

Dependabot regularly proposes digest updates for pinned Docker runtime base images in Radius while preserving the readable image tag next to each digest.

Proposed Fix

Update the authoritative sync configuration in radius-project/.github to add the docker ecosystem for the Radius Dockerfile directories, regenerate or sync .github/dependabot.yml, and verify Dependabot recognizes digest-pinned FROM references.

rad Version

N/A

Operating system

N/A

Additional context

Follow-up to #12922 and review feedback on #12984. Keep the dependency automation change separate from the Debian 13 pin so the source-of-truth organization configuration and generated Radius configuration can be updated together.

[radius-triage]

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[brooke-hamilton]

PR radius-project/.github#34 was added to the .github repo to update the dependabot sync file for the radius repo.
PR #12995 was synced to update the dependabot configuration for the Docker ecosystem.
Dependabot ran and produced this PR with container image updates: #13004