Add image build/push workflows, scoped to test/build/push only

Builds and pushes to Gitea's container registry on push to main
("latest"/"previous", to avoid accumulating per-commit tags) and on
git tag push (tagged to match). Deployment is intentionally out of
scope here -- that work is being split into a separate project.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitea/workflows/release.yml

@@ -0,0 +1,50 @@
+name: Release
+
+# Builds and pushes an image tagged to match the git tag that triggered this
+# run -- e.g. pushing tag "v1.2.3" produces code.aneur.in/<owner>/<repo>:v1.2.3.
+# Separate from build.yml's "latest"/"previous" tracking on push to main.
+# This project's CI/CD scope is intentionally just test/build/push --
+# deployment (including how a given tag actually gets deployed) is being
+# split into a separate project.
+#
+# Assumes tags are cut from commits already on main (and so already covered
+# by ci.yml's checks) -- this workflow doesn't run the test suite itself.
+#
+# Requires secrets.GITEA_TOKEN (auto-injected; needs the "packages"
+# permission scope to push images).
+
+on:
+  push:
+    tags: ['*']
+
+permissions:
+  contents: read
+  packages: write
+
+jobs:
+  build-and-push:
+    runs-on: ubuntu-latest
+    # Assumes the runner forwards the host's Docker socket into job
+    # containers -- see the matching note in build.yml if this can't reach
+    # a daemon.
+    container:
+      image: docker:cli
+      options: -v /var/run/docker.sock:/var/run/docker.sock
+    steps:
+      # actions/checkout is a JS action; docker:cli is Alpine-based and has
+      # no node on PATH by default (same issue fixed in ci.yml).
+      - name: Install Node
+        run: apk add --no-cache nodejs
+
+      - uses: actions/checkout@v4
+
+      - name: Log in to the container registry
+        env:
+          GITEA_TOKEN: ${{ secrets.GITEA_TOKEN }}
+        run: echo "$GITEA_TOKEN" | docker login code.aneur.in -u "${{ gitea.actor }}" --password-stdin
+
+      - name: Build and push the image
+        run: |
+          IMAGE="code.aneur.in/${{ gitea.repository }}:${{ gitea.ref_name }}"
+          docker build -t "$IMAGE" .
+          docker push "$IMAGE"