Move user code background

[mzmssg]

Make our monitor process as docker main process, to avoid user command trick

A know issue:
For killing job sceanario, yarn script will exit before docker script, so the yarn log aggregation may collect incomplete dockerContainerDebug.log

[coveralls]

Coverage increased (+0.02%) to 51.632% when pulling 86629d4 on zimiao/refactor_docker_script into b8ec568 on master.