Multiple intermittent test failures

[nbering]

There are a number of cases where Travis jobs will fail due to an error - usually things that manipulate files in Docker containers - but re-running the job will succeed.

I haven't yet been able to trace the issue to a common problem.

[nbering]

This seems statistically most likely to occur on 17.12, with test:

Docker::Image#insert_local when a directory is passed inserts the directory

But I'm still not sure whether the issue is the same underlying this test failure, and there is only one issue, or if we have multiple intermittent issues.

[nbering]

The pattern I've seen seeing in these failures is that they seem mostly to be related to file access of some kind or another. I haven't seen one, for example, fail to pull images or start a container. It's been about diffing the contents or a file, listing a directory's contents in a container, etc.