tests: Run ocis container as root for testsuite

For running the acceptance tests in docker we setup a volume that is
shared between the ocis and oc10/testsuite container. Since ocis needs
to be able to write to that volume we're switching it to run as root.

Closes #2497

tests/acceptance/docker/src/ocis-image.yml

@@ -1,3 +1,4 @@
 services:
   ocis-server:
     image: owncloud/ocis:$OCIS_IMAGE_TAG
+    user: root