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.runner.yml

@@ -0,0 +1,26 @@
+# Gitea act_runner — CI/CD executor for Gitea Actions
+#
+# Setup steps:
+#   1. Get a runner registration token from:
+#      https://devops.cloudhost.es/Malin/paste.es/settings/actions/runners
+#   2. Add it to your .env file:
+#      RUNNER_REGISTRATION_TOKEN=<token from Gitea>
+#   3. Start the runner:
+#      docker compose -f docker-compose.runner.yml up -d
+#
+# The runner registers itself automatically on first start.
+# After that, push to main and the docker.yml workflow will fire.
+
+services:
+    act-runner:
+        image: gitea/act_runner:latest
+        container_name: paste-es-act-runner
+        restart: unless-stopped
+        volumes:
+            - /var/run/docker.sock:/var/run/docker.sock
+            - ./data/runner:/data
+        environment:
+            - GITEA_INSTANCE_URL=https://devops.cloudhost.es
+            - GITEA_RUNNER_REGISTRATION_TOKEN=${RUNNER_REGISTRATION_TOKEN}
+            - GITEA_RUNNER_NAME=paste-es-runner
+            - GITEA_RUNNER_LABELS=ubuntu-latest:docker://node:20-bullseye