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/loki/Containerfile

@@ -1,9 +1,13 @@
 # Loki + a static busybox.
 #
-# grafana/loki is distroless: no shell, no wget, so the compose healthcheck
-# cannot run anything inside the container. This image drops a static
-# busybox into it for the healthcheck command only. All Loki behavior is
-# unchanged.
+# grafana/loki is distroless: no shell, no wget, so nothing can run inside the
+# container to check it. This image drops a static busybox into it for the
+# healthcheck. All Loki behavior is unchanged.
+#
+# The healthcheck lives HERE, in exec form, and not in the Quadlet unit: a
+# unit's HealthCmd reaches podman as CMD-SHELL, which needs /bin/sh — the one
+# thing this image does not have. Specifying it here is also what lets the
+# container report its own health instead of looking merely "Up" while wedged.
 #
 # Build on dankclaw:
 #   podman build -t franksdank/loki:latest -f dankclaw/loki/Containerfile .
@@ -15,4 +19,7 @@ FROM docker.io/library/alpine:latest AS tools
 RUN apk add --no-cache busybox-static
 
 FROM docker.io/grafana/loki:latest
-COPY --from=tools /bin/busybox.static /usr/local/bin/busybox
+COPY --from=tools /bin/busybox.static /usr/local/bin/busybox
+# Exec form (JSON array): no shell is involved.
+HEALTHCHECK --interval=30s --timeout=10s --start-period=30s --retries=3 \
+    CMD ["/usr/local/bin/busybox", "wget", "--spider", "-q", "http://localhost:3100/ready"]