build(docker): optimize image size and security

- add mysql client binaries for database operations
- reduce php extensions to production essentials
- bind mysql and phpmyadmin to localhost only
- replace php-fpm.conf with php.ini for upload limits

docker/php.ini

@@ -0,0 +1,2 @@
+post_max_size = 20M
+upload_max_filesize = 20M