Add Dockerfile, nginx config, and Gitea Actions CI workflow

Multi-stage Docker build produces an Nginx container serving the
static SPA. The CI workflow runs pnpm check on every push and
builds/pushes a Docker image on semver tags.

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

.gitea/workflows/ci.yml

@@ -0,0 +1,42 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+    tags: ["*"]
+  pull_request:
+    branches: [main]
+
+jobs:
+  check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: pnpm/action-setup@v4
+
+      - uses: actions/setup-node@v4
+        with:
+          node-version: 22
+          cache: pnpm
+
+      - run: pnpm install --frozen-lockfile
+      - run: pnpm check
+
+  build-image:
+    if: startsWith(github.ref, 'refs/tags/')
+    needs: check
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Log in to Gitea registry
+        run: echo "${{ secrets.REGISTRY_PASSWORD }}" | docker login git.bahamut.nitrix.one -u "${{ secrets.REGISTRY_USERNAME }}" --password-stdin
+
+      - name: Build and push
+        run: |
+          IMAGE=git.bahamut.nitrix.one/dostulata/initiative
+          TAG=${GITHUB_REF#refs/tags/}
+          docker build -t $IMAGE:$TAG -t $IMAGE:latest .
+          docker push $IMAGE:$TAG
+          docker push $IMAGE:latest