In commit 7980178d194bb21bde2e21858fbc969e4cc7eb3f (#10879) I added a
`usable?` class method to `VagrantPlugins::DockerProvider::Provider`.
However, commit 34e53a5a4b208edcc3e25c1f0e3aa0ef56e8d8d9 (#10890)
incorrectly changed it to an instance method. This rendered it
ineffective because it’s called on the class, not an instance. Change
it back to a class method.
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
5b4dcf9443