Fixes sanity tests (#10)

The sanity tests are failing with the following error:

exec /whereabouts version
error: enter: cannot provide -v/--verbose before "exec" subcommand

Pebble was recently updated, no longer allowing exec after --verbose
(which is in the entrypoint).

This fixes the sanity tests by using the --entrypoint docker run
parameter.

tests/sanity/test_whereabouts.py

@@ -21,9 +21,10 @@ def _run_in_docker(image, check_exit_code, *command):
             "run",
             "--rm",
             "-i",
+            "--entrypoint",
+            command[0],
             image,
-            "exec",
-            *command,
+            *command[1:],
         ],
         check=check_exit_code,
         capture_output=True,