By pruning machines that are in "unknown" status after each operation,
the Go code path exposed the fact that the Docker provider was not
updating the machine index during an "up" - leaving the state as
"unknown".
This is basically a bug within the Docker provider, so I think it's okay
to update the plugin code to correct this rather than working around the
issue in Go.
All we need to do is call `machine.state` instead of reaching through to
`machine.provider.state` while waiting for the container to be started.
That causes the extra logic for updating the machine index in
`machine.state` to fire.
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