Makefile: build image is a prerequisite of the shell target
Makefile: build image is a prerequisite of the shell target

Signed-off-by: Euan Harris <c3b6e83069c8e9e3af49a38fec6026be89559638@docker.com>

docker/libnetwork,mahak/libnetwork,vdemeester/libnetwork,abhi/libnetwork,vdemeester/libnetwork,medallia/libnetwork,docker/libnetwork,abhi/libnetwork,docker/libnetwork,vdemeester/libnetwork,abhi/libnetwork,medallia/libnetwork,mahak/libnetwork,mahak/libnetwork,medallia/libnetwork