cannot remove cron directory

docker run -it alpine:3.9 (also seems to effect earlier versions including 3.7)

```
/ # ls -lah /var/spool
total 12
drwxr-xr-x 3 root root 4.0K Jan 30 02:54 .
drwxr-xr-x 11 root root 4.0K Jan 30 02:54 ..
drwxr-xr-x 2 root root 4.0K Jan 30 02:54 cron
/ # ls -lah /var/spool/cron/
total 8
drwxr-xr-x 2 root root 4.0K Jan 30 02:54 .
drwxr-xr-x 3 root root 4.0K Jan 30 02:54 ..
lrwxrwxrwx 1 root root 13 Jan 30 02:54 crontabs -> /etc/crontabs
```

```
/ # rm -rf /var/spool/cron
rm: can't remove '/var/spool/cron': Invalid argument
```
From that point im left at a have working shell with just
`>
`
and cannot do much

Also deleting the crontabs doesnt seem to make any difference