fix(observability): give loki an exec-form healthcheck in its image

The loki unit came up 'unhealthy' after the quadlet cutover. Cause: a unit's
HealthCmd reaches podman as CMD-SHELL, i.e. /bin/sh -c, and the loki image is
distroless — no shell exists in it. The busybox command itself works (probed
by hand: /ready answers 'ready'); it was the shell wrapper that could not run.
podman does not accept an exec-form command through --health-cmd either: it
stores a JSON array as one literal argument.

The command therefore moves into dankclaw/loki/Containerfile as a
HEALTHCHECK ... CMD [...] exec form, which is what that image exists for — it
already ships the static busybox solely so a healthcheck has something to run.
podman reads the image's healthcheck when the unit declares none.

dankclaw/observability/quadlet/loki.container

@@ -43,11 +43,11 @@ Exec=-config.file=/etc/loki/loki-config.yaml
 PublishPort=127.0.0.1:3100:3100
 Volume=/opt/loki/config:/etc/loki
 Volume=/opt/loki/data:/loki
-HealthCmd=/usr/local/bin/busybox wget --spider -q http://localhost:3100/ready
-HealthInterval=30s
-HealthTimeout=10s
-HealthRetries=3
-HealthStartPeriod=30s
+# No HealthCmd here on purpose. A unit's HealthCmd reaches podman as
+# CMD-SHELL, which means `/bin/sh -c <command>` — and this image is
+# distroless, with no shell at all. The healthcheck therefore ships in the
+# image (dankclaw/loki/Containerfile) in exec form, where it also carries its
+# interval, timeout, start period and retry count.
 
 [Service]
 Restart=always