Don't mount code in a volume when running tests

An image is built anyway, so this is unnecessary. This makes it
possible to run the tests on a remote Docker daemon.

Signed-off-by: Ben Firshman <ben@firshman.co.uk>

script/test

@@ -9,7 +9,6 @@ docker build -t "$TAG" .
 docker run \
   --rm \
   --volume="/var/run/docker.sock:/var/run/docker.sock" \
-  --volume="$(pwd):/code" \
   -e DOCKER_VERSIONS \
   -e "TAG=$TAG" \
   --entrypoint="script/test-versions" \