Revert to docker build and bump runner label to v2.5.1

Temporarily revert to legacy `docker build` to bootstrap the
forgejo-runner v2.5.1 image (which includes buildx). Also update
the runner label to use v2.5.1 so subsequent builds use the new image.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.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 }}

argocd/manifests/forgejo-runner/external-secret.yaml

@@ -25,7 +25,7 @@ spec:
       data:
         FORGEJO_URL: "https://forge.ops.eblu.me"
         RUNNER_NAME: "k8s-runner"
-        RUNNER_LABELS: "k8s:docker://registry.ops.eblu.me/blumeops/forgejo-runner:v2.5.0"
+        RUNNER_LABELS: "k8s:docker://registry.ops.eblu.me/blumeops/forgejo-runner:v2.5.1"
         RUNNER_TOKEN: "{{ .runner_token }}"
   data:
   - secretKey: runner_token