Add Docker build and Gitea Actions CI for deployment

Multi-stage Alpine Dockerfile producing a standalone Next.js image
with the better-sqlite3 native addon included. Gitea workflow builds
on push to main and on v* tags, pushing to the Gitea registry using
the REGISTRY_URL variable and REGISTRY_TOKEN secret. Compose file
mounts ./data so alerts.db and settings.json persist across restarts.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,27 @@
+.git
+.gitea
+.github
+.claude
+.vscode
+.idea
+
+node_modules
+.next
+out
+build
+coverage
+
+data
+.env*
+
+README.md
+*.md
+!package.json
+Dockerfile
+docker-compose*.yml
+.dockerignore
+
+*.log
+.DS_Store
+*.tsbuildinfo
+next-env.d.ts