Prepend the rootfs mounts to the user specified runtime.mounts.
Prepend the rootfs mounts to the user specified runtime.mounts.

User specified mounts should be able to rely on the rootfs being mounted, in
particular for a writeable container they should expect the writeable overlay
to already be in place.

Signed-off-by: Ian Campbell <a8db091655a7f47ecb9c39a2d278e9662e440dc0@docker.com>

JohnnyLeone/linuxkit,JohnnyLeone/linuxkit,deitch/linuxkit,yankcrime/linuxkit,justincormack/linuxkit,djs55/linuxkit,linuxkit/linuxkit,furious-luke/linuxkit,linuxkit/linuxkit,linuxkit/linuxkit,furious-luke/linuxkit,deitch/linuxkit,JohnnyLeone/linuxkit,yankcrime/linuxkit,djs55/linuxkit,rn/linuxkit,YuPengZTE/linuxkit,linuxkit/linuxkit,deitch/linuxkit,justincormack/linuxkit,linuxkit/linuxkit,justincormack/linuxkit,rn/linuxkit,yankcrime/linuxkit,furious-luke/linuxkit,justincormack/linuxkit,konstruktoid/linuxkit,YuPengZTE/linuxkit,justincormack/linuxkit,justincormack/tool,djs55/linuxkit,YuPengZTE/linuxkit,konstruktoid/linuxkit,justincormack/linuxkit,deitch/linuxkit,rn/linuxkit,yankcrime/linuxkit,YuPengZTE/linuxkit,YuPengZTE/linuxkit,deitch/linuxkit,konstruktoid/linuxkit,JohnnyLeone/linuxkit,furious-luke/linuxkit,konstruktoid/linuxkit,rn/linuxkit,yankcrime/linuxkit,djs55/linuxkit,konstruktoid/linuxkit,rn/linuxkit,JohnnyLeone/linuxkit,yankcrime/linuxkit,djs55/linuxkit,YuPengZTE/linuxkit,furious-luke/linuxkit,konstruktoid/linuxkit