Add OCI source and version labels to images (#975)

Adds `org.opencontainers.image.source` and `org.opencontainers.image.version` labels to all three image variants (`basic`, `dind`, `dind-rootless`).

- `source` lets tools like renovate retrieve release notes from the source repo.
- `version` exposes the build version on the image itself.

Both `release-tag` and `release-nightly` workflows pass `VERSION` as a build arg so the label reflects the actual git tag (or `git describe` output for nightly).

---
This PR was written with the help of Claude Opus 4.7

---------

Reviewed-on: https://gitea.com/gitea/runner/pulls/975
Reviewed-by: Nicolas <bircni@icloud.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-committed-by: silverwind <me@silverwind.io>

.gitea/workflows/release-nightly.yml

@@ -71,6 +71,11 @@ jobs:
       - name: Echo the tag
         run: echo "${{ env.DOCKER_ORG }}/runner:nightly${{ matrix.variant.tag_suffix }}"
 
+      - name: Get Meta
+        id: meta
+        run: |
+          echo REPO_VERSION=$(git describe --tags --always | sed 's/-/+/' | sed 's/^v//') >> $GITHUB_OUTPUT
+
       - name: Build and push
         uses: docker/build-push-action@v7
         with:
@@ -83,3 +88,5 @@ jobs:
           push: true
           tags: |
             ${{ env.DOCKER_ORG }}/runner:nightly${{ matrix.variant.tag_suffix }}
+          build-args: |
+            VERSION=${{ steps.meta.outputs.REPO_VERSION }}