Bootstrap buildx: revert to docker build, bump runner to v2.5.1 (#148)

## Summary
- Temporarily revert composite action to `docker build` so we can build the runner image (chicken-and-egg: current runner v2.5.0 doesn't have buildx)
- Bump runner label to `v2.5.1` so after sync the new runner image (with buildx) gets used

## Deployment plan
1. Merge this PR
2. Tag `forgejo-runner-v2.5.1` — builds with legacy `docker build` (one last time)
3. Sync forgejo-runner in ArgoCD to pick up the v2.5.1 label
4. Follow-up PR: switch action back to `docker buildx build`
5. Tag `nettest-v0.12.0` to verify buildx works end-to-end

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/148

.forgejo/actions/build-push-image/action.yaml

@@ -30,7 +30,7 @@ runs:
       shell: bash
       run: |
         echo "Building ${{ inputs.image_name }}:${{ inputs.version }}"
-        docker buildx build \
+        docker build \
           --tag ${{ inputs.image_name }}:${{ inputs.version }} \
           --file ${{ inputs.context }}/${{ inputs.dockerfile }} \
           ${{ inputs.context }}