feat: complete Gitea Actions CI workflow for container builds

- Fix trigger/env path: Helm/ -> helm/ (was wrong case)
- Add image_yq_path and dockerfile to matrix so each container uses its
  own values.yaml key and Dockerfile path
- Fix yq paths: .django.image.* -> .frontend.image/.backend.image for
  repo, .containers.version for tag (single source of truth)
- Add file: param to docker/build-push-action (Dockerfiles are in
  frontend/ and backend/, not repo root)
- values.yaml: add registry prefix to image fields so k8s pulls from
  git.baumann.gr; quote containers.version; drop per-component tag
  fields (containers.version is now the single tag source)
- Deployment templates: use .containers.version for image tag

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

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

@@ -0,0 +1,171 @@
+name: Build containers when image tags change
+
+on:
+  push:
+    # Uncomment/adjust once working:
+    # branches: [ master ]
+    paths:
+      - "helm/shorefront/values.yaml"
+      - "frontend/Dockerfile"
+      - "backend/Dockerfile"
+      - ".gitea/workflows/build-containers-on-demand.yml"
+
+jobs:
+  build-if-image-changed:
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - container_name: shorefront-frontend
+            image_yq_path: .frontend.image
+            expected_repo: git.baumann.gr/adebaumann/shorefront-frontend
+            dockerfile: frontend/Dockerfile
+            build_context: .
+            description: shorefront frontend
+          - container_name: shorefront-backend
+            image_yq_path: .backend.image
+            expected_repo: git.baumann.gr/adebaumann/shorefront-backend
+            dockerfile: backend/Dockerfile
+            build_context: .
+            description: shorefront backend
+
+    env:
+      DEPLOY_FILE: "helm/shorefront/values.yaml"
+      CONTAINER_NAME: ${{ matrix.container_name }}
+      EXPECTED_REPO: ${{ matrix.expected_repo }}
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 2
+
+      - name: Determine base commit
+        id: base
+        shell: bash
+        run: |
+          set -euo pipefail
+          if git rev-parse --verify -q HEAD~1 >/dev/null; then
+            echo "base=$(git rev-parse HEAD~1)" >> "$GITHUB_OUTPUT"
+          else
+            echo "base=$(git hash-object -t tree /dev/null)" >> "$GITHUB_OUTPUT"
+          fi
+
+      - name: Install yq
+        shell: bash
+        run: |
+          set -euo pipefail
+          YQ_VER=v4.44.3
+          curl -sL "https://github.com/mikefarah/yq/releases/download/${YQ_VER}/yq_linux_amd64" -o /usr/local/bin/yq
+          chmod +x /usr/local/bin/yq
+          yq --version
+
+      - name: Read ${{ matrix.description }} image from Helm values
+        id: img
+        shell: bash
+        run: |
+          set -euo pipefail
+          file="${DEPLOY_FILE}"
+          expected_repo="${EXPECTED_REPO}"
+
+          echo "========================================"
+          echo "Reading image from: $file"
+          echo "Expected repo: $expected_repo"
+          echo "========================================"
+
+          new_repo=$(yq -r '${{ matrix.image_yq_path }}' "$file")
+          new_tag=$(yq -r '.containers.version' "$file")
+          new_image="${new_repo}:${new_tag}"
+
+          if git cat-file -e "${{ steps.base.outputs.base }}:$file" 2>/dev/null; then
+            old_repo=$(git show "${{ steps.base.outputs.base }}:$file" | yq -r '${{ matrix.image_yq_path }}' || true)
+            old_tag=$(git show "${{ steps.base.outputs.base }}:$file" | yq -r '.containers.version' || true)
+            old_image="${old_repo}:${old_tag}"
+          else
+            old_image="<none>"
+          fi
+
+          echo "Old image: ${old_image}"
+          echo "New image: ${new_image}"
+
+          if [ -z "${new_repo:-}" ] || [ "$new_repo" = "null" ]; then
+            echo "ERROR: Could not read ${{ matrix.image_yq_path }} from $file"
+            exit 1
+          fi
+
+          if [[ "$new_repo" != "$expected_repo" ]]; then
+            echo "ERROR: Found image repo \"$new_repo\" but expected \"$expected_repo\""
+            exit 1
+          fi
+
+          registry="$(echo "$new_repo" | awk -F/ '{print $1}')"
+
+          {
+            echo "new_image=$new_image"
+            echo "new_repo=$new_repo"
+            echo "new_tag=$new_tag"
+            echo "registry=$registry"
+          } >> "$GITHUB_OUTPUT"
+
+      - name: Check if image exists on registry
+        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="${{ gitea.actor }}"
+          registry_password="${{ secrets.REGISTRY_TOKEN }}"
+
+          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 $registry_host"
+
+          manifest_url="https://${registry_host}/v2/${image_path}/manifests/${new_tag}"
+
+          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.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.check_image.outputs.exists == 'false'
+        uses: docker/setup-buildx-action@v3
+
+      - name: Log in to registry
+        if: steps.check_image.outputs.exists == 'false'
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ steps.img.outputs.registry }}
+          username: ${{ gitea.actor }}
+          password: ${{ secrets.REGISTRY_TOKEN }}
+
+      - name: Build and push ${{ matrix.description }} (exact tag from Helm values)
+        if: steps.check_image.outputs.exists == 'false'
+        uses: docker/build-push-action@v6
+        with:
+          context: ${{ matrix.build_context }}
+          file: ${{ matrix.dockerfile }}
+          push: true
+          tags: |
+            ${{ steps.img.outputs.new_image }}
+            ${{ steps.img.outputs.new_repo }}:latest
+          labels: |
+            org.opencontainers.image.source=${{ gitea.repository }}
+            org.opencontainers.image.revision=${{ gitea.sha }}
+          cache-from: type=gha
+          cache-to: type=gha,mode=max