Add some wrappers to support at least the default uid mapping

The busybox adduser and addgroup commands are ignored by docker,
as it avoids symlinks, but they are not compatible so this is
kind of a good idea.

See https://github.com/docker/docker/blob/master/pkg/idtools/usergroupadd_linux.go

However, these do not yet deal with creating mapping ranges in /etc/subuid and
/etc/subgid, so initially we provide a set that match the default docker name
so at least docker daemon --userns-remap=default should work.

Signed-off-by: Justin Cormack <justin.cormack@docker.com>

alpine/packages/userns/groupadd

@@ -0,0 +1,3 @@
+#!/bin/sh
+
+exec /usr/sbin/addgroup $@