fix(devadmin): enforce PHP 5

The docker image richarvey/nginx-php-fpm that we're using has switched
its default to PHP 7, breaking the way our poweradmin is set up.

devadmin/Dockerfile

@@ -1,4 +1,4 @@
-FROM richarvey/nginx-php-fpm
+FROM richarvey/nginx-php-fpm:php5
 
 RUN apk add --no-cache \
 	php5-gettext \