Rebuild docker image when Dockerfile is changed
    
    freshmaker is able to build a docker image when changed Dockerfile is
    detected. freshmaker listens on fedmsg bus and handles message from
    topic git.receive. When Dockerfile is changed, freshmaker will start to
    build docker image in Koji immediately. Note that, freshmaker does not
    wait for the task to finish, it continues to handle next coming message
    instead.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>