busybox-static for healthchecks; alloy: rely on the default UI listener

- alpine's /bin/busybox is musl-dynamic and cannot execute in the
  distroless loki image (missing loader); busybox-static links in
- alloy 1.x refuses to configure the ui service from a config block;
  the default UI already listens on 127.0.0.1:12345 where the
  healthcheck probes

shared/alloy/config.alloy

@@ -5,12 +5,9 @@ logging {
   level = "info"
 }
 
-// Local listener so the container healthcheck can probe us.
-// Alloy >= 1.0 renamed the server component to ui.
-ui {
-  host = "0.0.0.0"
-  port = 12345
-}
+// The alloy UI service listens on 127.0.0.1:12345 by default; the
+// in-container healthcheck probes it there. alloy 1.x refuses to
+// configure the ui service from a config block, so there is no block.
 
 // Ship logs to Loki (container name on the podman network).
 loki.write "default" {