Increase PHP memory limit

- sometimes the iPeer user page is blank but ok after reload.
  caused by PHP out-of-memory if there are too many users. increasing
  memory limit from the default 128MB to 256MB

docker/php.ini

@@ -1,4 +1,5 @@
 display_errors = Off
 error_reporting = E_ALL & ~E_DEPRECATED
 date.timezone = America/Vancouver
+memory_limit=256M
 zend_extension=/usr/local/lib/php/extensions/no-debug-non-zts-20170718/opcache.so