adduser mangling /etc/passwd at new user shell

Caught me off guard, this. Definitely a bug.

```
FROM gliderlabs/alpine:latest
Step 5 : ENV lxcuser "lxcmedia" lxcuid "1024" lxcgroup "users" lxcgid "100"
Step 15 : RUN adduser -h /home/$lxcuser -g "lxc-media user" -u $lxcuid -G $lxcgroup -D $lxcuser
```

This translates into the (triple-checked) command `adduser -h /home/lxcmedia -g "lxc-media user" -u 1024 -G 100 -D lxcmedia` which should create the user with the default shell. Instead, what it does to /etc/passwd is this:

```
nobody:x:65534:65534:nobody:/:/sbin/nologin
lxcmedia:x:1024:100:lxc-media user:/home/lxcmedia:obody
```

This is busybox 8586 - https://bugs.busybox.net/show_bug.cgi?id=8586 - should be fixed in 1.24.1. Confirmed no reproduction on 1.24.1 with buildroot, but alpine is reproducing reliably.