Create group for user in sonic-slave if does absent (#98)

sonic-slave/Dockerfile

@@ -91,6 +91,8 @@ ARG user
 ARG uid
 ARG guid
 
+RUN groupadd -f -r -g $guid g$user
+
 RUN useradd $user -u $uid -g $guid -d /var/$user -m -s /bin/bash
 
 RUN gpasswd -a $user docker