fix: restore working docker/build-push-action (#213)

Kaniko approach doesn't work on unprivileged runners because even
'docker run' to start the kaniko container requires unshare capabilities.

Restored the docker/build-push-action@v5 which was working before.
The Dockerfile still has Gleam removed - only the build method changed.

Reviewed-on: graham/towerops-web#213

.forgejo/workflows/production.yaml

@@ -113,34 +113,24 @@ jobs:
           echo "tag=${TAG}" >> $GITHUB_OUTPUT
           echo "Full image tag: ${TAG}"
 
-      - name: Build and push with kaniko
-        run: |
-          IMAGE_TAG="${{ steps.tag.outputs.tag }}"
+      - name: Log in to Container Registry
+        uses: https://github.com/docker/login-action@v3
+        with:
+          registry: ${{ secrets.REGISTRY_URL }}
+          username: ${{ secrets.REGISTRY_USER }}
+          password: ${{ secrets.REGISTRY_PASSWORD }}
 
-          # Create kaniko config for registry auth
-          mkdir -p /tmp/kaniko
-          cat > /tmp/kaniko/config.json <<EOF
-          {
-            "auths": {
-              "${{ secrets.REGISTRY_URL }}": {
-                "auth": "$(echo -n "${{ secrets.REGISTRY_USER }}:${{ secrets.REGISTRY_PASSWORD }}" | base64)"
-              }
-            }
-          }
-          EOF
-
-          # Run kaniko to build and push
-          docker run --rm \
-            -v $(pwd):/workspace \
-            -v /tmp/kaniko:/kaniko/.docker \
-            gcr.io/kaniko-project/executor:latest \
-            --context /workspace \
-            --dockerfile k8s/Dockerfile \
-            --destination ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${IMAGE_TAG} \
-            --destination ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:production \
-            --build-arg MIX_ENV=prod \
-            --cache=false \
-            --cleanup
+      - name: Build and push Docker image
+        uses: https://github.com/docker/build-push-action@v5
+        with:
+          context: .
+          file: k8s/Dockerfile
+          push: true
+          tags: |
+            ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:${{ steps.tag.outputs.tag }}
+            ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}:production
+          build-args: |
+            MIX_ENV=prod
 
       - name: Update deployment manifest
         run: |