fix: healthcheck — use IPv4 and busybox-compatible nc

The Alpine runtime stage uses BusyBox nc, which has no -z flag, and
'localhost' resolves to ::1 while finger only binds 0.0.0.0:79. Both
combined caused every healthcheck to fail (35k+ failing streak on
mammut). Use '-w 1 127.0.0.1 79 < /dev/null' instead.

docker-compose.yml

@@ -9,7 +9,7 @@ services:
       - ./users:/var/finger/users
     restart: unless-stopped
     healthcheck:
-      test: ["CMD", "nc", "-z", "localhost", "79"]
+      test: ["CMD-SHELL", "nc -w 1 127.0.0.1 79 < /dev/null || exit 1"]
       interval: 30s
       timeout: 10s
       retries: 3