feat(ci): add Gitea Actions workflow for building and pushing to GHCR

Add .gitea/workflows/build.yaml that:
- Builds Docker image on push to master
- Pushes to ghcr.io/unchainedio/manoon-headless
- Tags with commit SHA and 'latest'
- Updates k8s/kustomization.yaml with new image tag
- Commits and pushes the tag update back to repo

Requires Gitea Actions runner to be configured.

.gitea/workflows/build.yaml

@@ -0,0 +1,60 @@
+name: Build and Push to GHCR
+
+on:
+  push:
+    branches: [master, main]
+
+env:
+  REGISTRY: ghcr.io
+  IMAGE_NAME: ${{ github.repository }}
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    
+    steps:
+    - name: Checkout repository
+      uses: actions/checkout@v4
+
+    - name: Set up Docker Buildx
+      uses: docker/setup-buildx-action@v3
+
+    - name: Log in to Container Registry
+      uses: docker/login-action@v3
+      with:
+        registry: ${{ env.REGISTRY }}
+        username: ${{ github.actor }}
+        password: ${{ secrets.GITHUB_TOKEN }}
+
+    - name: Extract metadata
+      id: meta
+      uses: docker/metadata-action@v5
+      with:
+        images: ${{ env.REGISTRY }}/${{ env.IMAGE_NAME }}
+        tags: |
+          type=sha,prefix=,suffix=,format=short
+          type=raw,value=latest,enable={{is_default_branch}}
+
+    - name: Build and push Docker image
+      uses: docker/build-push-action@v5
+      with:
+        context: .
+        push: true
+        tags: ${{ steps.meta.outputs.tags }}
+        labels: ${{ steps.meta.outputs.labels }}
+        cache-from: type=gha
+        cache-to: type=gha,mode=max
+
+    - name: Update kustomization.yaml
+      run: |
+        COMMIT_SHA=${{ github.sha }}
+        SHORT_SHA=${COMMIT_SHA:0:7}
+        sed -i "s|newTag: .*|newTag: ${SHORT_SHA}|" k8s/kustomization.yaml
+
+    - name: Commit and push changes
+      run: |
+        git config --local user.email "gitea-actions[bot]@users.noreply.gitea.io"
+        git config --local user.name "gitea-actions[bot]"
+        git add k8s/kustomization.yaml
+        git diff --quiet && git diff --staged --quiet || git commit -m "deploy: update image to ${SHORT_SHA} [skip ci]"
+        git push