Adjusted behavior to match docker-compose when both an image and a build stanza are present in the same service.
Adjusted behavior to match docker-compose when both an image and a build stanza are present in the same service.

Signed-off-by: Daniel Steen <2591e5f46f28d303f9dc027d475a5c60d8dea17a@steen.blue>

weitenghuang/libcompose,Azure22/libcompose,dweomer/docker-libcompose,docker/libcompose,jainvipin/libcompose,vdemeester/libcompose,jainvipin/libcompose,Azure22/libcompose,vdemeester/libcompose,docker/libcompose,vito-c/libcompose,dweomer/docker-libcompose,weitenghuang/libcompose,vito-c/libcompose