support docker on EMR 6.x AMIs

[coyotemarin]

EMR now has explicit support for Docker on 6.x AMIs. mrjob should integrate with this

[coyotemarin]

Probably should add the following options:

• docker_image: name of the docker image
• docker_registry: hostname of the docker registry (defaults to centos, which is Docker Hub)
• docker_client_config: the config file on HDFS, needed to log into ECR
• docker_mounts: a list of volumes to mount inside docker. Defaults to ['/etc/passwd:/etc/passwd:ro']

[coyotemarin]

Magic mrjob can provide:

• adding the right lines to emr_config
• adding the right variables to cmdenv
• copying the client config into HDFS

[coyotemarin]

We'll need to bootstrap mrjob in setup if we want to be able to access it inside Docker.