Add image digest arguments to config serialization
Add image digest arguments to config serialization

Add arguments for image digests in the config.serialize module to
optionally pin images to digests, like bundles.

Signed-off-by: King Chung Huang <c7a206d540065858ce9913e5b8b61b905e19f510@mac.com>

swoopla/compose,vdemeester/compose,schmunk42/compose,hoogenm/compose,jrabbit/compose,shin-/compose,sdurrheimer/compose,dnephin/compose,thaJeztah/compose,swoopla/compose,shin-/compose,funkyfuture/docker-compose,schmunk42/compose,hoogenm/compose,vdemeester/compose,thaJeztah/compose,jrabbit/compose,funkyfuture/docker-compose,sdurrheimer/compose,dnephin/compose