Add Promtail to ship Docker container logs to Loki

Loki was running but had no log shipper — no labels or logs were
visible in Grafana. Adds Promtail to scrape all Docker container
logs and push them to Loki.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

infrastructure/docker-compose.yml

@@ -128,6 +128,17 @@ services:
       - /var/lib/docker/:/var/lib/docker:ro
       - /dev/disk/:/dev/disk:ro
 
+  promtail:
+    image: grafana/promtail:latest
+    restart: unless-stopped
+    volumes:
+      - /var/log:/var/log:ro
+      - /var/lib/docker/containers:/var/lib/docker/containers:ro
+      - ./promtail/promtail-config.yml:/etc/promtail/config.yml:ro
+    command: ["-config.file=/etc/promtail/config.yml"]
+    depends_on:
+      - loki
+
   prometheus:
     image: prom/prometheus:latest
     restart: unless-stopped