Correct the flag `enableshell` to `enableusershell`
Correct the flag `enableshell` to `enableusershell`

`enableshell` is not defined in the repo anywhere, hence looks like a typo.
mehulsbhatt/dockersh,mehulsbhatt/dockersh,CS497-F15/dockersh,Yelp/dockersh,beni55/dockersh,chenrui2014/dockersh,Yelp/dockersh,beni55/dockersh,pugna0/dockersh,pugna0/dockersh,CS497-F15/dockersh,chenrui2014/dockersh