Use a go build cache to speed up builds.

With a docker build cache already primed with the build image I am seeing
`time make build -f docker.Makefile DOCKER_BUILDKIT=1 GO_BUILD_CACHE=n` takes
more than 1 minute.

By contrast `time make build -f docker.Makefile DOCKER_BUILDKIT=1
GO_BUILD_CACHE=y` takes less than 10s with a hot cache irrespective of whether
the source tree has changed

Signed-off-by: Ian Campbell <ijc@docker.com>

docker.Makefile

@@ -10,7 +10,11 @@ LINTER_IMAGE_NAME = docker-cli-lint$(IMAGE_TAG)
 CROSS_IMAGE_NAME = docker-cli-cross$(IMAGE_TAG)
 VALIDATE_IMAGE_NAME = docker-cli-shell-validate$(IMAGE_TAG)
 E2E_IMAGE_NAME = docker-cli-e2e$(IMAGE_TAG)
+GO_BUILD_CACHE ?= y
 MOUNTS = -v "$(CURDIR)":/go/src/github.com/docker/cli
+ifeq ($(GO_BUILD_CACHE),y)
+MOUNTS += -v "docker-cli-dev-cache:/root/.cache/go-build"
+endif
 VERSION = $(shell cat VERSION)
 ENVVARS = -e VERSION=$(VERSION) -e GITCOMMIT -e PLATFORM