specify that docker_compose role may not be included twice

Our docker compose role is configured via ansible variables in a way
that overwrites each other if it is activated twice with different
variable assignments (unless e.g. a role that just installs more
packages).
To prevent misuse of the role, the allow_duplicates key of the role
meta is set to false. This should make ansible complain if the role
is included twice for a single host.

roles/docker_compose/meta/main.yaml

@@ -1,3 +1,4 @@
 ---
+allow_duplicates: false
 dependencies:
   - role: docker