* Add ability to use digests for images referenced in registry

Add script /build/scripts/write_image_digests.sh, which will rewrite all
image references in registry to use the current digest for each image
tag.

Requires adding skopeo to the dockerfiles, in order to find image
digests.

Option can be enabled via `./build.sh` parameter `--use-digests` or by
using docker build arg "USE_DIGESTS=true"

Signed-off-by: Angel Misevski <amisevsk@redhat.com>