Add shared.gemfile to Dockerfile
Add shared.gemfile to Dockerfile

Fixes #388
thredded/thredded,jayroh/thredded,thredded/thredded,jayroh/thredded,jvoss/thredded,jayroh/thredded,jayroh/thredded,thredded/thredded,thredded/thredded,jvoss/thredded,jvoss/thredded,jvoss/thredded