As someone who's used both Gitlab, Github Enterprise, and Stash in a corporate environment, I have to admit I prefer GHE, for the features it shares with core Github, if nothing else.
My only gripe w/ GHE is the lack of branch-based ACLs. If we want to implement a Github Flow (https://guides.github.com/introduction/flow/index.html) methodology, but also have automatic branch detection and testing in our CI (Jenkins/Bamboo/whatever), we end up having to assign all users the ability to create branches in the given repository (which, in turn, gives them the perms to push to master). I'm actually quite surprised that this hasn't been implemented yet in GHE; I can understand why GH wouldn't have it, but GHE (because of the nature of its user base) really should.
That issue aside, the UI in GHE is simply superior to GL and Stash.
I'm pretty sure GHE has the ability to disable pushing to the default branch (which is set on a per-project basis, but by default it's master). But I think this is a per-instance setting (unless that's been changed recently).
So while everyone would have the ability to push to other branches, they wouldn't be able to touch master outside of pull requests, which I think is what you want anyways.
I'm extremely surprised that Packer (http://www.packer.io) hasn't received more attention in the Docker arena. Its Docker builder, coupled with any of its provisioners, makes it a pretty attractive alternative to the simplistic layout and limitations of Dockerfiles.
Packer currently doesn't snapshot the build at each step, so you don't get a history that you can checkout or push from. It's a feature that's on the way, but it's a pretty big one that some teams need.
We use packer for a lot of other things though and it's great.
