configure Loki for local development (#26997)

* configure Loki for local development

Extend our docker compose setup to include Loki (along with promtail), scraping `logs/*.log` to ingest development logs as well as all logs from running containers.

While we're in here, teach Prometheus to scrape metrics from Docker containers too.

* tweak promtail labels

* document extra services in config.mk

* update home.json

server/build/docker-compose.yml

@@ -44,6 +44,14 @@ services:
     extends:
         file: docker-compose.common.yml
         service: grafana
+  loki:
+    extends:
+        file: docker-compose.common.yml
+        service: loki
+  promtail:
+    extends:
+        file: docker-compose.common.yml
+        service: promtail
 
   start_dependencies:
     image: mattermost/mattermost-wait-for-dep:latest