nginx-ingress-controller is built twice by docker-build target

[porridge]

porridge@kielonek:~/projects/go/src/k8s.io/ingress$ make test-e2e 
go get github.com/onsi/ginkgo/ginkgo
2017/03/30 12:38:12 e2e.go:276: Running: build-release
make[1]: Entering directory `/usr/local/google/home/porridge/projects/go/src/k8s.io/ingress'
make -C controllers/nginx build
make[2]: Entering directory `/usr/local/google/home/porridge/projects/go/src/k8s.io/ingress/controllers/nginx'
rm -f rootfs/nginx-ingress-controller
CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo \
                -ldflags "-s -w -X k8s.io/ingress/controllers/nginx/pkg/version.RELEASE=0.9.0-beta.3 -X k8s.io/ingress/controllers/nginx/pkg/version.COMMIT=git-e385c05 -X k8s.io/ingress/controllers/nginx/pkg/ve
rsion.REPO=git@github.com:porridge/ingress.git" \
                -o rootfs/nginx-ingress-controller k8s.io/ingress/controllers/nginx/pkg/cmd/controller
make[2]: Leaving directory `/usr/local/google/home/porridge/projects/go/src/k8s.io/ingress/controllers/nginx'
make -C controllers/nginx container
make[2]: Entering directory `/usr/local/google/home/porridge/projects/go/src/k8s.io/ingress/controllers/nginx'
rm -f rootfs/nginx-ingress-controller
CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo \
                -ldflags "-s -w -X k8s.io/ingress/controllers/nginx/pkg/version.RELEASE=0.9.0-beta.3 -X k8s.io/ingress/controllers/nginx/pkg/version.COMMIT=git-e385c05 -X k8s.io/ingress/controllers/nginx/pkg/ve
rsion.REPO=git@github.com:porridge/ingress.git" \
                -o rootfs/nginx-ingress-controller k8s.io/ingress/controllers/nginx/pkg/cmd/controller
gcloud docker -- build --pull -t gcr.io/google_containers/nginx-ingress-controller:0.9.0-beta.3 rootfs
[...]

Seems to be a waste of time...

[porridge]

This is because the top-level docker-build target depends on controllers while the container target in controllers/nginx/Makefile also depends on build.

And there is a similarly superfluous dependency between docker-push and docker-build.

[k8s-reviewable]

This change is 

[coveralls]

Changes Unknown when pulling ed65647 on porridge:make-deps into ** on kubernetes:master**.

[aledbf]

/lgtm

[aledbf]

@porridge thanks!