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>

docker/php.ini

@@ -0,0 +1,19 @@
+; ---- PHP limits ----
+max_execution_time = 0
+max_input_time = -1
+max_input_vars = 3000
+memory_limit = 512M
+post_max_size = 32M
+upload_max_filesize = 16M
+default_socket_timeout = 120
+
+; Recommended defaults
+date.timezone = UTC
+display_errors = Off
+error_reporting = E_ALL & ~E_DEPRECATED & ~E_STRICT
+
+; Opcache
+opcache.enable=1
+opcache.enable_cli=1
+opcache.validate_timestamps=1
+opcache.revalidate_freq=2