ci: Replace GitLab CI with Gitea Actions workflow

Switched from GitLab CI to Gitea Actions for container builds.
Triggers on version tags, pushes to Gitea Container Registry.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

.gitea/workflows/build-container.yml

@@ -0,0 +1,45 @@
+# Gitea Actions workflow for StarPunk
+# Builds and pushes container images on version tags
+
+name: Build Container
+
+on:
+  push:
+    tags:
+      - 'v[0-9]+.[0-9]+.[0-9]+'
+
+jobs:
+  build:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
+
+      - name: Extract registry URL
+        id: registry
+        run: |
+          # Extract hostname from server URL (remove protocol)
+          REGISTRY_URL=$(echo "${{ github.server_url }}" | sed 's|https://||' | sed 's|http://||')
+          echo "url=${REGISTRY_URL}" >> $GITHUB_OUTPUT
+
+      - name: Login to Gitea Container Registry
+        uses: docker/login-action@v3
+        with:
+          registry: ${{ steps.registry.outputs.url }}
+          username: ${{ github.actor }}
+          password: ${{ secrets.GITHUB_TOKEN }}
+
+      - name: Build and push
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          file: ./Containerfile
+          push: true
+          tags: |
+            ${{ steps.registry.outputs.url }}/${{ github.repository }}:${{ github.ref_name }}
+            ${{ steps.registry.outputs.url }}/${{ github.repository }}:latest
+          cache-from: type=gha
+          cache-to: type=gha,mode=max