Add Dockerfile back
    
    Dockerfile was deleted in commit 232f9a3, now it's back without any
    internal things as it would be helpful for upstream users.
    
    The image will be available at https://quay.io/repository/exd-guild-compose/cts
    and the test image will use it as base image.
    
    Using `--trusted-host pypi.org` in pip to avoid ssl verification issue.
    
    Signed-off-by: Haibo Lin <hlin@redhat.com>