Pass environment through to docker process

`os.Setenv` will set an environment variable for the current
process and all child processes, then not specifying `Env` on
`exec.Command` will cause all environment variables to be
inherited.

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Merge pull request #10 from orchardup/pass-environment-through-to-docker

Pass environment through to docker process