Support all DockerInfo fields

With this change, the workflow also was altered.
You now parse the various compound keys into their
respective field types, and pass in the parse results.

All 'optional' fields in the DockerInfo message are
an 'Option' type, where 'None' will use the default in
the message.

With mesos 0.21, there will be more fields to add, but they
can all follow a simmilar pattern to the existing fields.