Add support for referring to images by digest in docker-compose.yml

[lmakarov]

Pulling by image digest is supported since docker 1.6.0 (moby/moby#11109)

docker pull busybox@sha256:38a203e1986cf79639cfb9b2e1d6e773de84002feea2d4eb006b52004ee8502d

However putting an image digest for a service in docker-compose.yml leads to

Pulling busybox (busybox@sha256:38a203e1986cf79639cfb9b2e1d6e773de84002feea2d4eb006b52004ee8502d)...
Invalid repository name (busybox@sha256), only [a-z0-9-_.] are allowed

Sample docker-compose.yml

service:
  image: busybox@sha256:38a203e1986cf79639cfb9b2e1d6e773de84002feea2d4eb006b52004ee8502d

docker version
Client version: 1.7.0
Client API version: 1.19
Go version (client): go1.4.2
Git commit (client): 0baf609
OS/Arch (client): darwin/amd64
Server version: 1.7.0
Server API version: 1.19
Go version (server): go1.4.2
Git commit (server): 0baf609
OS/Arch (server): linux/amd64

docker-compose --version
docker-compose version: 1.3.1
CPython version: 2.7.9
OpenSSL version: OpenSSL 1.0.1j 15 Oct 2014

[bfirsh]

Thanks for the suggestion, @lmakarov. This is absolutely something we should support.