Support specifying image by digest in compose files.

[axisofentropy]

docker pull minio/minio@sha256:5eedfe2636306b83f5758ba8c7b94b7e161446ed7db038d35575ebf74204bd54

This command works, and demonstrates specifying an image by its unique hash. We should support this in our compose files as well.

When I try now, even with 'quotes', I get this error: undefined method values' for nil:NilClass` That error should probably be improved as well.

This was an attempt to work around #310.