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>

.gitea/workflows/docker.yml

@@ -0,0 +1,44 @@
+name: Build and Push Docker Image
+
+on:
+    push:
+        branches:
+            - main
+    workflow_dispatch:
+
+jobs:
+    build:
+        name: Build & push to Gitea registry
+        runs-on: ubuntu-latest
+
+        steps:
+            - name: Checkout
+              uses: actions/checkout@v4
+
+            - name: Set up Docker Buildx
+              uses: docker/setup-buildx-action@v3
+
+            - name: Login to Gitea Container Registry
+              uses: docker/login-action@v3
+              with:
+                  registry: devops.cloudhost.es
+                  username: ${{ gitea.actor }}
+                  password: ${{ secrets.GITEA_TOKEN }}
+
+            - name: Extract image metadata
+              id: meta
+              uses: docker/metadata-action@v5
+              with:
+                  images: devops.cloudhost.es/malin/paste.es
+                  tags: |
+                      type=raw,value=latest,enable={{is_default_branch}}
+                      type=sha,prefix=,format=short
+
+            - name: Build and push
+              uses: docker/build-push-action@v6
+              with:
+                  context: .
+                  push: true
+                  tags: ${{ steps.meta.outputs.tags }}
+                  cache-from: type=gha
+                  cache-to: type=gha,mode=max