Great to have a ready-to-go docker image for building! Was just about to do one myself, but first took a look around (just didn't want to reinvent the wheel...). One thing though, it's not necessary to rsync /etc/skel to the home of cmbuild. If you call useradd with the switch -m, /home/cmbuild will get created by useradd including the copying of the skeleton files. You can even take it a step further and add your scripts like startup.sh and folders to /etc/skel before doing the useradd, then you don't need to do the permission fixing in the end (just as an idea).
EDIT: And you're missing vim in your image (just my favourite command line editor...)