Setting up a shared repository for users with no shell login

[alex mitchell]

I apologize if this is a FAQ, but I haven't been able to find this in the
docs.

What I'd like to do is set up my shared repository on an Ubuntu-based
server, and let my project team use bzr to push/pull/commit files to/from
the repository. However, I don't want them to be able to log in and browse
around and execute commands in the filesystem using ssh or sftp.

Is there any way to do this? I've tried restricting login by setting the
users' shell to /bin/false, which stops the users from logging in, but this
also blocks bzr from connecting using sftp. Can I block users from logging
in, but still allow bzr to have access to read/write to the files within the
repository?

thanks!
Alex Mitchell
alexm at nus.edu.sg