New Workflow to check if image is present on gitea server

.gitea/workflows/build-containers-on-demand.yml

@@ -231,12 +231,51 @@ jobs:
         if: steps.img.outputs.changed != 'true'
         run: echo "${{ matrix.description }} image tag unchanged; skipping build."
 
-      - name: Set up Buildx
+      - name: Check if image exists on registry
         if: steps.img.outputs.changed == 'true'
+        id: check_image
+        shell: bash
+        run: |
+          set -euo pipefail
+          new_repo="${{ steps.img.outputs.new_repo }}"
+          new_tag="${{ steps.img.outputs.new_tag }}"
+          registry_user="${{ secrets.REGISTRY_USER }}"
+          registry_password="${{ secrets.REGISTRY_PASSWORD }}"
+
+          # Extract registry host and image name
+          registry_host=$(echo "$new_repo" | cut -d/ -f1)
+          image_path=$(echo "$new_repo" | cut -d/ -f2-)
+
+          echo "Checking if $new_repo:$new_tag exists on registry $registry_host"
+
+          # Use Docker Registry API v2 to check manifest
+          # Format: https://registry/v2/{image_path}/manifests/{tag}
+          manifest_url="https://${registry_host}/v2/${image_path}/manifests/${new_tag}"
+
+          # Check with authentication
+          http_code=$(curl -s -o /dev/null -w "%{http_code}" \
+            -u "${registry_user}:${registry_password}" \
+            -H "Accept: application/vnd.docker.distribution.manifest.v2+json,application/vnd.docker.distribution.manifest.list.v2+json" \
+            "$manifest_url" || echo "000")
+
+          if [ "$http_code" = "200" ]; then
+            echo "Image already exists on registry (HTTP $http_code)"
+            echo "exists=true" >> "$GITHUB_OUTPUT"
+          else
+            echo "Image does not exist on registry (HTTP $http_code)"
+            echo "exists=false" >> "$GITHUB_OUTPUT"
+          fi
+
+      - name: Skip if image already exists
+        if: steps.img.outputs.changed == 'true' && steps.check_image.outputs.exists == 'true'
+        run: echo "${{ matrix.description }} image ${{ steps.img.outputs.new_image }} already exists on registry; skipping build."
+
+      - name: Set up Buildx
+        if: steps.img.outputs.changed == 'true' && steps.check_image.outputs.exists == 'false'
         uses: docker/setup-buildx-action@v3
 
       - name: Log in to registry
-        if: steps.img.outputs.changed == 'true'
+        if: steps.img.outputs.changed == 'true' && steps.check_image.outputs.exists == 'false'
         uses: docker/login-action@v3
         with:
           registry: ${{ steps.img.outputs.registry }}
@@ -244,7 +283,7 @@ jobs:
           password: ${{ secrets.REGISTRY_PASSWORD }}
 
       - name: Build and push ${{ matrix.description }} (exact tag from deployment)
-        if: steps.img.outputs.changed == 'true'
+        if: steps.img.outputs.changed == 'true' && steps.check_image.outputs.exists == 'false'
         uses: docker/build-push-action@v6
         with:
           context: ${{ matrix.build_context }}