version pin the docker image in docker-compose (#1945)

While the [documentation](https://github.com/grafana/loki/blob/master/docs/installation/docker.md) does version pin the links to specific versions of the docker-compose file, the file it docker-compose file does use the `latest` tag for loki.

For an example of issues this causes see #1941

production/docker-compose.yaml

@@ -5,7 +5,7 @@ networks:
 
 services:
   loki:
-    image: grafana/loki:latest
+    image: grafana/loki:1.4.1
     ports:
       - "3100:3100"
     command: -config.file=/etc/loki/local-config.yaml
@@ -13,7 +13,7 @@ services:
       - loki
 
   promtail:
-    image: grafana/promtail:latest
+    image: grafana/promtail:1.4.1
     volumes:
       - /var/log:/var/log
     command: -config.file=/etc/promtail/docker-config.yaml