feat: Gitea Actions CI/CD pipeline with container registry builds

- .gitea/workflows/docker.yml: builds and pushes image to
  devops.cloudhost.es/malin/paste.es on every push to main,
  tags as :latest and :<short-sha>, uses GHA layer cache
- docker-compose.yml: updated to pull pre-built image from Gitea
  container registry instead of building locally
- docker-compose.build.yml: new override file for local dev builds
  (docker compose -f docker-compose.yml -f docker-compose.build.yml up --build)
- docker-compose.runner.yml: act_runner service — mount docker socket,
  auto-registers with RUNNER_REGISTRATION_TOKEN from .env
- .env.example: added RUNNER_REGISTRATION_TOKEN section
- README.md: document CI/CD setup, runner registration, and updated
  quick-start to use registry pull workflow

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

docker-compose.build.yml

@@ -0,0 +1,11 @@
+# Local build override — use this during development instead of pulling from the registry.
+#
+# Usage:
+#   docker compose -f docker-compose.yml -f docker-compose.build.yml up -d --build
+
+services:
+    paste-es:
+        build:
+            context: .
+            dockerfile: Dockerfile
+        image: devops.cloudhost.es/malin/paste.es:latest