Use better defaults for vagrant user in Dockerfile

The adduser script is recommended over useradd in Debian.
Sets Bash as default shell instead of /bin/sh, in particular.

Closes tildes-community/tildes-cf#18

See merge request tildes-community/tildes-cf!7

docker/Dockerfile-for-vagrant

@@ -1,10 +1,8 @@
 FROM debian:12
 ENV container docker
 
-RUN useradd --create-home vagrant \
-  && echo "vagrant:vagrant" | chpasswd \
-  && groupadd wheel \
-  && usermod -a -G wheel vagrant
+RUN adduser --quiet vagrant \
+  && echo "vagrant:vagrant" | chpasswd
 
 # allow vagrant to login
 RUN cd ~vagrant \