feat: add release Docker image and Gitea Actions workflow

Add Dockerfile.release using mise to install toolchain, Gitea Actions
workflow with snapshot dry-run on main pushes and full release on tags,
and mise release-docker task for local testing.

.gitea/workflows/release.yaml

@@ -0,0 +1,36 @@
+name: Release
+
+on:
+  push:
+    branches:
+      - main
+    tags:
+      - "v*"
+
+jobs:
+  release:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Build release image
+        run: docker build -f Dockerfile.release -t gitnow-release .
+
+      - name: Snapshot (dry run)
+        if: startsWith(github.ref, 'refs/tags/') == false
+        run: |
+          docker run --rm \
+            -v "${{ github.workspace }}:/build" \
+            -w /build \
+            gitnow-release release-snapshot
+
+      - name: Release
+        if: startsWith(github.ref, 'refs/tags/')
+        run: |
+          docker run --rm \
+            -v "${{ github.workspace }}:/build" \
+            -w /build \
+            -e GITEA_TOKEN=${{ secrets.GITEA_TOKEN }} \
+            gitnow-release release