Use image tagged with master in docker-compose.yaml (#77)

This pull request makes the image used in the `docker-compose.yaml` the same as the one used in the `README.md`. The currently used loki image in the `docker-compose.yaml` makes the service crash.

Signed-off-by: Fabian Mersch <fm@deltacity.net>

docs/docker-compose.yaml

@@ -5,7 +5,7 @@ networks:
 
 services:
   loki:
-    image: grafana/loki:master-8fa9461
+    image: grafana/loki:master
     ports:
       - "3100:3100"
     volumes: