A task that builds and pushes docker containers
    
    If the repo/commit in question contain a Dockerfile,
    .taskotron_task_dockerbuild.json file, and login/password is provided
    from Vault, then a docker container is built based on the commit and
    pushed.
    
    The .taskotron_task_dockerbuild.json file looks like this:
    {
       "docker_project_name": "resultsdb",
       "docker_credentials_bucket_uuid": "1e77148e-64ab-11e8-8f59-525400ee7c53",
       "release_branch": "master"
    }
    
    The images are docker-tagged like this
      docker.io/fedoraqa/$PROJECT:$SHA
      docker.io/fedoraqa/$PROJECT:$BRANCH
    
    `docker_project_name` is the $PROJECT
    `release_branch` is replaced with `latest` for the purpose of tagging
    `docker_credentials_bucket_uuid` is the UUID of Vault provided bucket
    containing the credentials
    
    The Vault bucket has to provide `dockerhub_login` and
    `dockerhub_password` keys.