Update docker.md (#5213)
Update docker.md (#5213)

Create the containers in detached mode and name the promtail container "promtail".
Without this fix the first container must be stopped before the second one is started since they are running in foreground.
The promtail container didn't have any "--name" flag so docker assigns a random name to it.
grafana/loki,grafana/loki,grafana/loki,grafana/loki,grafana/loki,grafana/loki