Set correct env variables for `docker exec` commands

I made a test case for this in https://github.com/docker/docker/pull/9174

because `LoadContainerEnvironment` is already run, os.Environ() will return the
correct environment variables for the exec command (i.e. removed duplicated
envs, set HOME for user etc...)

Docker-DCO-1.1-Signed-off-by: Daniel, Dao Quang Minh <dqminh89@gmail.com> (github: dqminh)