While trying to get docker image pre-caching to work we couldn't get a
docker daeomon to run within the chrooted environment. However we got
docker running with the help of bwrap outside of the chrooted
environment. The only option so far for this is the block-device.d
phase. But this has the problem that it runs after the image size has
been calculated. This leads to broken builds if the docker images
being pulled are big.
This can be solved by adding a post-root.d phase that runs outside the
chroot but before the image size calculation.
Change-Id: I36c2a81e2d9f5069f18ce5b0d52c5f1c7212c3ae
eff5b2312b