Add Gitea CI/CD workflows for automated Docker builds

Implements automated Docker image building and publishing to registry:

- build.yml: Main workflow that builds and pushes Docker images to registry
  - Triggers on push to master branch
  - Only builds when image with current version doesn't exist
  - Uses Docker BuildKit with layer caching for faster builds
  - Tags images with both version number and 'latest'

- check_image_version.yml: Reusable workflow to verify image existence
  - Reads version from package.json
  - Uses lightweight manifest inspection (no image download)
  - Returns image_exists and version as outputs

- check_package_version.yml: Reusable workflow to detect version changes
  - Compares version between commits
  - Handles edge cases (first commit, missing package.json)
  - Includes validation for version extraction failures

All workflows include proper error handling and clear logging.

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

.gitea/workflows/build.yml

@@ -0,0 +1,58 @@
+name: Build and Push Docker Image
+
+on:
+  push:
+    branches:
+      - master
+
+jobs:
+  # Verifies if Docker image with current version already exists in registry
+  # This prevents rebuilding the same version but allows pulls and version changes
+  # to always trigger new builds. Uses lightweight manifest inspect (no download)
+  build_web_app__check_image_version:
+    uses: ./.gitea/workflows/check_image_version.yml
+    with:
+      workspacePath: './web-app'
+      imageName: 'utility-bills-tracker'
+      registryUrl: 'registry.budakova.org'
+      registryUsername: ${{ vars.PROFILE_REGISTRY_USERNAME }}
+      registryNamespace: 'knee-cola'
+    secrets:
+      registryToken: ${{ secrets.PROFILE_REGISTRY_TOKEN }}
+
+  # Builds and pushes Docker image to registry if:
+  # - Image with current version doesn't exist in registry
+  # This prevents rebuilding the same version unnecessarily
+  build_web_app:
+    needs: [build_web_app__check_image_version]
+    if: needs.build_web_app__check_image_version.outputs.image_exists == 'false'
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Login to Registry
+        # Gitea automatically provides these secrets:
+        # - `vars.REGISTRY_USERNAME` - defined as action variable in **repo settings**
+        # - `secrets.REGISTRY_TOKEN` - defined as action secret in **repo settings**
+        #                              created in user settings as personal access token with `write:packages` scope
+        # - `vars.PROFILE_REGISTRY_USERNAME` - defined as action variable in **profile settings**
+        # - `secrets.PROFILE_REGISTRY_TOKEN` - defined as action secret in **profile settings**
+        #                                      created in user settings as personal access token with `write:packages` scope
+        run: |
+          echo "${{ secrets.PROFILE_REGISTRY_TOKEN }}" | docker login registry.budakova.org -u "${{ vars.PROFILE_REGISTRY_USERNAME }}" --password-stdin
+
+      - name: Build and push Docker image
+        uses: docker/build-push-action@v5
+        with:
+          context: ./web-app
+          push: true
+          tags: |
+            registry.budakova.org/knee-cola/utility-bills-tracker:${{ needs.build_web_app__check_image_version.outputs.version }}
+            registry.budakova.org/knee-cola/utility-bills-tracker:latest
+          cache-from: type=registry,ref=registry.budakova.org/knee-cola/utility-bills-tracker:buildcache
+          cache-to: type=registry,ref=registry.budakova.org/knee-cola/utility-bills-tracker:buildcache,mode=max