feat: add Docker setup exposing app on port 7876

- Dockerfile: php:apache base, installs PHP extensions + Composer,
  configures Apache to listen on port 7876, bundles app code,
  runs composer install at build time
- docker-compose.yml: web service (port 7876) + MariaDB LTS with
  healthcheck; writable volume for DB persistence
- docker/entrypoint.sh: generates .env from environment variables
  at container start, fixes runtime permissions
- docker/php.ini: tuned PHP limits (512M memory, 32M upload)
- .dockerignore: excludes .git, vendor, logs, cache, old docker dir
- .env.example: template for required environment variables

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

.dockerignore

@@ -0,0 +1,17 @@
+.git
+.github
+vendor/
+.env
+.env.*
+logs/*.log
+logs/*.txt
+cache/*
+!cache/.gitkeep
+domain-monitor-docker/
+*.bak
+*.backup
+*.old
+*.tmp
+.DS_Store
+Thumbs.db
+node_modules/