Use skopeo for registry push (Docker 27 manifest compat)

Docker 27's manifest format has compatibility issues with zot registry.
Skopeo handles manifest conversion correctly.

Still simpler than the old Tailscale sidecar approach - just save to
tarball and use skopeo copy.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

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

@@ -1,5 +1,8 @@
 name: 'Build and Push Image'
-description: 'Build a container image with Docker and push to registry.ops.eblu.me'
+description: 'Build a container image with Docker and push to registry.ops.eblu.me using skopeo'
+
+# Note: Uses skopeo for push because Docker 27's manifest format has compatibility
+# issues with zot registry. Skopeo handles manifest conversion correctly.
 
 inputs:
   context:
@@ -26,19 +29,32 @@ runs:
     - name: Build image
       shell: bash
       run: |
-        echo "Building ${{ inputs.registry }}/${{ inputs.image_name }}:${{ inputs.version }}"
+        echo "Building ${{ inputs.image_name }}:${{ inputs.version }}"
         docker build \
-          --tag ${{ inputs.registry }}/${{ inputs.image_name }}:${{ inputs.version }} \
+          --tag ${{ inputs.image_name }}:${{ inputs.version }} \
-          --tag ${{ inputs.registry }}/${{ inputs.image_name }}:${{ github.sha }} \
           --file ${{ inputs.context }}/${{ inputs.dockerfile }} \
           ${{ inputs.context }}
 
     - name: Push to registry
       shell: bash
       run: |
+        echo "Saving image to tarball..."
+        docker save ${{ inputs.image_name }}:${{ inputs.version }} -o /tmp/image.tar
+
+        echo "Installing skopeo..."
+        apk add --no-cache skopeo >/dev/null 2>&1 || apt-get update && apt-get install -y skopeo >/dev/null 2>&1 || true
+
         echo "Pushing ${{ inputs.registry }}/${{ inputs.image_name }}:${{ inputs.version }}"
-        docker push ${{ inputs.registry }}/${{ inputs.image_name }}:${{ inputs.version }}
+        skopeo copy \
-        docker push ${{ inputs.registry }}/${{ inputs.image_name }}:${{ github.sha }}
+          docker-archive:/tmp/image.tar \
+          docker://${{ inputs.registry }}/${{ inputs.image_name }}:${{ inputs.version }}
+
+        echo "Pushing ${{ inputs.registry }}/${{ inputs.image_name }}:${{ github.sha }}"
+        skopeo copy \
+          docker-archive:/tmp/image.tar \
+          docker://${{ inputs.registry }}/${{ inputs.image_name }}:${{ github.sha }}
+
+        rm -f /tmp/image.tar
 
     - name: Summary
       shell: bash