Fix startup on userns-remap built image

When the image was built on or pulled by a dockerd running with userns-remap the root
filesystem will be owned by the remapped UID.  When that image is run explicitly in the
host's user namespace the ownership of the files on disk will be based on the userns-remapped
UIDs.  This is generally fine, except where an executable is SUID, in which case, which is the
case for the s6-overlay-preinit tool, so we must detect the situation and set the ownership of the
relevant tools.

See:
- just-containers#309
- moby/moby#28986
- https://docs.docker.com/engine/security/userns-remap/#disable-namespace-remapping-for-a-container

builder/overlay-rootfs/init

@@ -10,4 +10,8 @@
 
 /bin/importas -D /usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin PATH PATH
 export PATH ${PATH}
-/etc/s6/init/init-stage1 $@
+
+foreground {
+    backtick -n REMAPPED_ROOT_UID { /usr/bin/stat -c '%u' /bin/s6-overlay-preinit }
+    if { s6-test $REMAPPED_ROOT_UID -ne 0 } /bin/s6-chown -u 0 /bin/s6-overlay-preinit
+} /etc/s6/init/init-stage1 $@