Fix broken Dockerfile and docker-startup.sh for correct user and group usage

Signed-off-by: Marco Dickert <marco@misterunknown.de>

Dockerfile

@@ -14,6 +14,13 @@ RUN apk add --no-cache libbz2 libzip libcap sudo && \
 # allow php binary to bind ports <1000, even if $USER != root
 RUN /usr/sbin/setcap CAP_NET_BIND_SERVICE=+eip /usr/local/bin/php
 
+# remove unnecessary users
+RUN deluser xfs && \
+    deluser www-data
+
+# sudo: workaround for https://bugzilla.redhat.com/show_bug.cgi?id=1773148
+RUN echo "Set disable_coredump false" > /etc/sudo.conf
+
 # prepare files
 RUN rm -rf /var/www/html && \
     mkdir -p /usr/local/share/webapps/ifm && \