I am using RHEL 6.6 (2.6.32-504.el6.x86_64) and when I try to fire up bash inside a running container with Docker exec I get this error:
$ docker exec -it 2acd76422b4f bash
2014/11/21 23:04:11 Error response from daemon: Unsupported: Exec is not supported by the lxc driver
$ docker version
Client version: 1.3.1
Client API version: 1.15
Go version (client): go1.3.3
Git commit (client): 4e9bbfa
OS/Arch (client): linux/amd64
Server version: 1.3.1
Server API version: 1.15
Go version (server): go1.3.3
Git commit (server): 4e9bbfa