docker: allow import docker in reset to fail

* if the user hasn't installed the `[docker]` extra, they won't have
  `docker` to `import`, so don't make `import docker` required in the
  docker driver's `reset`.
* fixes #166

src/molecule_plugins/docker/driver.py

@@ -255,7 +255,11 @@ def sanity_checks(self):
         self._passed_sanity = True
 
     def reset(self):
-        import docker
+        # maybe use self.sanity_check instead?
+        try:
+            import docker
+        except ImportError:
+            return
 
         client = docker.from_env()
         for c in client.containers.list(filters={"label": "owner=molecule"}):