Docker workflow: version as {jellyfin-version}-{rolling-number}, run on release/manual only

[Copilot]

The Docker publish workflow used git semver tags for image versioning and ran on every push to master. It now derives the image tag from the Jellyfin base image version in the Dockerfile with an auto-incrementing build counter (e.g. 10.11.6-1), and only triggers on GitHub releases or manual dispatch.

Versioning

• Extracts Jellyfin version from FROM jellyfin/jellyfin:X.Y.Z in docker/Dockerfile at build time
• Queries GHCR via gh api for existing {version}-N tags; increments the highest found, defaulting to 1 on first publish
• Dots in the version string are escaped before regex matching to prevent false matches (e.g. 101116 won't match 10.11.6)
• Removed docker/metadata-action — tags are now computed explicitly

10.11.6-1   ← first publish
10.11.6-2   ← subsequent publish, same base image
10.11.7-1   ← after Dockerfile bumped to 10.11.7

Triggers

• Removed push: branches: master trigger
• Workflow now runs only on release (type: released) and workflow_dispatch
• latest tag is applied on release events (previously on master push)

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