variabilized portainer's repo, allowing for portainer BE to be selected

docker-compose.yml

@@ -1,7 +1,7 @@
 ---
 services:
   portainer:
-    image: portainer/portainer-ce:${PORTAINER_VERSION}
+    image: ${PORTAINER_REPO}:${PORTAINER_VERSION}
     restart: unless-stopped
     command: -H unix:///var/run/docker.sock
     labels: