Update Dockerfile (#2162)

the first time, I ran apache2 container with other things such as MariaDB & PHPMyAdmin
the second time, I decided to run nginx instead of apache2 and I faced an error that said "adduser: group 'www-data' in use", so I solved and shared it to others.

nginx/Dockerfile

@@ -18,6 +18,10 @@ RUN apk update \
     && apk add --no-cache openssl \
     && apk add --no-cache bash
 
+RUN set -x ; \
+    addgroup -g 82 -S www-data ; \
+    adduser -u 82 -D -S -G www-data www-data && exit 0 ; exit 1
+
 ARG PHP_UPSTREAM_CONTAINER=php-fpm
 ARG PHP_UPSTREAM_PORT=9000