contrib/rootfs-builder: add /proc, /dev, /sys in rootfs

The rootfs needs to have /proc, /dev, /sys pre-created because the
runtime might not be able to create those, specially in user namespaces.

Signed-off-by: Alban Crequy <[email protected]>

Author: alban

contrib/rootfs-builder/rootfs-files

@@ -1,3 +1,6 @@
+./proc
+./dev
+./sys
 ./bin/busybox
 ./etc/group
 ./etc/passwd