feat: Add docker-compose for Gitea Actions runner

Runs gitea/act_runner on a CI/CD server, connecting to a remote Gitea
instance. Mounts the host Docker socket so build.sh can call docker
buildx inside workflow jobs.

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

docker-compose.yml

@@ -0,0 +1,28 @@
+# Docker Compose for Gitea Actions runner (CI/CD server).
+# Connects to a remote Gitea instance to run the build-and-release workflow.
+#
+# Usage:
+#   1. Get a runner token:  Gitea Admin Panel → Actions → Runners → Create new runner
+#   2. Copy .env.example to .env and fill in GITEA_INSTANCE_URL and RUNNER_REGISTRATION_TOKEN
+#   3. docker compose up -d
+
+services:
+  runner:
+    image: gitea/act_runner:latest
+    container_name: gitea-runner
+    environment:
+      - GITEA_INSTANCE_URL=${GITEA_INSTANCE_URL}
+      - GITEA_RUNNER_REGISTRATION_TOKEN=${RUNNER_REGISTRATION_TOKEN}
+      - GITEA_RUNNER_NAME=${RUNNER_NAME:-vaultwarden-builder}
+      # Map ubuntu-latest to an act-compatible image.
+      # build.sh calls docker buildx inside the job, so the host Docker socket
+      # is mounted below to make Docker available in job containers.
+      - GITEA_RUNNER_LABELS=ubuntu-latest:docker://catthehacker/ubuntu:act-22.04
+    volumes:
+      - runner-data:/data
+      # Required: build.sh runs docker buildx build inside the workflow job.
+      - /var/run/docker.sock:/var/run/docker.sock
+    restart: unless-stopped
+
+volumes:
+  runner-data: