Update mkimage-arch.sh

mktemp creates a root directory ("/") with permissions set to 700. Default should be 755 so other users in the container can access its subdirs (e.g http user for nginx for /srv/http/test/index.html).

Docker-DCO-1.1-Signed-off-by: Samuel Andaya <samuel@andaya.net> (github: sandaya)
Merge pull request #3897 from sandaya/master

Update mkimage-arch.sh to set proper permissions on rootfs directory