observability: run prometheus/loki/grafana as 1000:1000 (rootless uid mapping)

Container root maps to a subuid that owns nothing under /opt, so the
data-dir writes fail. Container uid 1000 is the invoking user in
rootless podman.

dankclaw/compose.yaml

@@ -103,6 +103,9 @@ services:
     image: docker.io/prom/prometheus:latest
     container_name: prometheus
     restart: unless-stopped
+    # Rootless: container uid 1000 maps to the invoking user, so the
+    # container can write its /opt/prometheus/data bind mount.
+    user: "1000:1000"
     command:
       - --config.file=/etc/prometheus/prometheus.yml
       - --storage.tsdb.path=/prometheus
@@ -123,6 +126,9 @@ services:
     image: docker.io/grafana/grafana:latest
     container_name: grafana
     restart: unless-stopped
+    # Rootless: container uid 1000 maps to the invoking user, so the
+    # container can write its /opt/grafana/data bind mount.
+    user: "1000:1000"
     environment:
       GF_SERVER_ROOT_URL: "https://grafana.franksdank.cloud"
       GF_SECURITY_ADMIN_USER: "${GRAFANA_ADMIN_USER}"
@@ -144,6 +150,9 @@ services:
     image: docker.io/grafana/loki:latest
     container_name: loki
     restart: unless-stopped
+    # Rootless: container uid 1000 maps to the invoking user, so the
+    # container can write its /opt/loki/data bind mount.
+    user: "1000:1000"
     command: ["-config.file=/etc/loki/loki-config.yaml"]
     ports:
       - "3100:3100"