Support remote docker hosts on OS X.

    Support remote docker hosts on OS X.

    This commit brings two main changes, notably:

    Two new options that can be set as environment variables

    - DOCKER_OPTS: any arbitrary set of docker options. Example: --tlsverify
    - DOCKER_NATIVE: This forces the use of the native docker available.
                     This is most useful if you're on OS X and do not want
                     to use boot2docker.

    Now uses 'docker cp' instead of tar piping to transfer files. This
    currently must be done by copying the binaries off of the docker volume
    and into a local filesystem (/tmp) before a docker cp is done. This
    workaround will no longer be necessary after bug fix
    https://github.com/docker/docker/pull/8509 makes it into stable.

    This was necessary because the tar | tar method was creating corrupted
    archives on OS X even with the < /dev/null workaround.