Docker cache takes into account things like filesize, hash of file,
and even timestamp and access times when determining whether to use
the cache or not. This modifies the build script to adjust all the
timestamps to 0 epoch. Since we dont care about timestamps this allows
us to make far better use of the cache, including when downloading and
creating tarballs for source builds.
Change-Id: Id4287cf1fcaa2de63feaab9e6f4ebbd4defdb768
Partially-Implements: blueprint build-script
49a9fba23e