Replace debian-based image with alpine-based.

Remove supervisor usage. Cron now runs as separate service.
CI for backend now performs without Docker.

docker/php/php.dev.ini

@@ -0,0 +1,5 @@
+error_reporting = E_ALL;
+display_errors = On;
+
+# Disable Opcache caching
+opcache.validate_timestamps = 1;