added INCREMENTAL_BUILD option to Makefile to speed up rebuilds

Before:
=======
$ time make -C build -j4 build-images
real    2m33.772s
user    0m1.302s
sys     0m0.889s

Incremental re-builds:
=======
$ time make -C build -j4 build-controller-image INCREMENTAL_BUILD=1
real    0m1.837s
user    0m0.241s
sys     0m0.085s

$ time make -C build -j4 build-agones-sdk-image INCREMENTAL_BUILD=1
real    0m5.352s
user    0m0.447s
sys     0m0.244s

$ time make -C build -j4 build-ping-image INCREMENTAL_BUILD=1
real    0m1.318s
user    0m0.198s
sys     0m0.063s

$ time make -C build -j4 build-images INCREMENTAL_BUILD=1
real    0m5.506s
user    0m1.177s
sys     0m0.617s

build/Makefile

@@ -75,6 +75,11 @@ sidecar_tag = $(REGISTRY)/agones-sdk:$(VERSION)
 ping_tag = $(REGISTRY)/agones-ping:$(VERSION)
 
 go_version_flags = -ldflags "-X agones.dev/agones/pkg.Version=$(VERSION)"
+
+# by default perform full rebuild unless INCREMENTAL_BUILD is set
+go_rebuild_flags = -a
+# zip compression level
+zip_flags = -9
 DOCKER_RUN ?= docker run --rm $(common_mounts) -e "KUBECONFIG=/root/.kube/$(kubeconfig_file)" $(DOCKER_RUN_ARGS) $(build_tag)
 #    ___  ____    ___            _           _
 #   / _ \/ ___|  |_ _|_ __   ___| |_   _  __| | ___
@@ -101,6 +106,13 @@ ifdef DOCKER_RUN
 	ensure-build-image += ensure-build-image
 endif
 
+ifdef INCREMENTAL_BUILD
+	# keep a cache of files built by Go across docker invocations in a local directory.
+	common_mounts += -v $(CURDIR)/.gocache:/root/.cache/go-build
+	go_rebuild_flags = 
+	zip_flags = -0
+endif
+
 #   _____                    _
 #  |_   _|_ _ _ __ __ _  ___| |_ ___
 #    | |/ _` | '__/ _` |/ _ \ __/ __|
@@ -195,7 +207,7 @@ uninstall: $(ensure-build-image)
 build-controller-binary: $(ensure-build-image)
 	docker run --rm -e "CGO_ENABLED=0" $(common_mounts) $(build_tag) go build \
 	-tags $(GO_BUILD_TAGS) -o $(mount_path)/cmd/controller/bin/controller \
-	-a $(go_version_flags) -installsuffix cgo $(agones_package)/cmd/controller
+	$(go_rebuild_flags) $(go_version_flags) -installsuffix cgo $(agones_package)/cmd/controller
 
 # Lint the go source code.
 # use LINT_TIMEOUT to manipulate the linter timeout
@@ -213,15 +225,24 @@ push-controller-image: $(ensure-build-image)
 	docker push $(controller_tag)
 
 # build the static binary for the gamesever sidecar
-build-agones-sdk-binary: $(ensure-build-image)
+build-agones-sdk-binary: $(ensure-build-image) build-agones-sdk-binary-linux build-agones-sdk-binary-windows build-agones-sdk-binary-darwin 
+	docker run --rm $(common_mounts) -w $(mount_path)/cmd/sdk-server/bin/ $(build_tag) zip $(zip_flags) \
+		agonessdk-server-$(VERSION).zip sdk-server.darwin.amd64 sdk-server.linux.amd64 sdk-server.windows.amd64.exe
+
+# build the static binary for the gamesever sidecar for Linux
+build-agones-sdk-binary-linux: $(ensure-build-image)
 	docker run --rm -e "CGO_ENABLED=0" $(common_mounts) $(build_tag) go build \
-		-o $(mount_path)/cmd/sdk-server/bin/sdk-server.linux.amd64 -a $(go_version_flags) -installsuffix cgo $(agones_package)/cmd/sdk-server
+		-o $(mount_path)/cmd/sdk-server/bin/sdk-server.linux.amd64 $(go_rebuild_flags) $(go_version_flags) -installsuffix cgo $(agones_package)/cmd/sdk-server
+
+# build the static binary for the gamesever sidecar for Darwin (macOS)
+build-agones-sdk-binary-darwin: $(ensure-build-image)
 	docker run --rm -e "GOOS=darwin" -e "GOARCH=amd64" $(common_mounts) $(build_tag) go build \
-		-o $(mount_path)/cmd/sdk-server/bin/sdk-server.darwin.amd64 $(go_version_flags) $(agones_package)/cmd/sdk-server
+		-o $(mount_path)/cmd/sdk-server/bin/sdk-server.darwin.amd64 $(go_rebuild_flags) $(go_version_flags) $(agones_package)/cmd/sdk-server
+
+# build the windows binary for the gamesever sidecar for Windows
+build-agones-sdk-binary-windows: $(ensure-build-image)
 	docker run --rm -e "GOOS=windows" -e "GOARCH=amd64" $(common_mounts) $(build_tag) go build \
-		-o $(mount_path)/cmd/sdk-server/bin/sdk-server.windows.amd64.exe $(go_version_flags) $(agones_package)/cmd/sdk-server
-	docker run --rm $(common_mounts) -w $(mount_path)/cmd/sdk-server/bin/ $(build_tag) zip \
-		agonessdk-server-$(VERSION).zip sdk-server.darwin.amd64 sdk-server.linux.amd64 sdk-server.windows.amd64.exe
+		-o $(mount_path)/cmd/sdk-server/bin/sdk-server.windows.amd64.exe $(go_rebuild_flags) $(go_version_flags) $(agones_package)/cmd/sdk-server
 
 # Build the image for the gameserver sidecar
 build-agones-sdk-image: $(ensure-build-image) build-agones-sdk-binary
@@ -231,7 +252,7 @@ build-agones-sdk-image: $(ensure-build-image) build-agones-sdk-binary
 build-ping-binary: $(ensure-build-image)
 	docker run --rm -e "CGO_ENABLED=0" $(common_mounts) $(build_tag) go build \
 	-tags $(GO_BUILD_TAGS) -o $(mount_path)/cmd/ping/bin/ping \
-	-a $(go_version_flags) -installsuffix cgo $(agones_package)/cmd/ping
+	$(go_rebuild_flags) $(go_version_flags) -installsuffix cgo $(agones_package)/cmd/ping
 
 # Pushes up the ping image
 push-ping-image: $(ensure-build-image)
@@ -360,7 +381,7 @@ do-release:
 	cp $(agones_path)/sdks/cpp/bin/agonessdk-$(RELEASE_VERSION)-runtime-linux-arch_64.tar.gz $(agones_path)/release
 	cp $(agones_path)/sdks/cpp/bin/agonessdk-$(RELEASE_VERSION)-dev-linux-arch_64.tar.gz $(agones_path)/release
 	cp $(agones_path)/sdks/cpp/bin/agonessdk-$(RELEASE_VERSION)-src.zip $(agones_path)/release
-	cd $(agones_path) &&  zip -r ./release/agones-install-$(RELEASE_VERSION).zip ./README.md ./install ./LICENSE
+	cd $(agones_path) &&  zip $(zip_flags) -r ./release/agones-install-$(RELEASE_VERSION).zip ./README.md ./install ./LICENSE
 	$(MAKE) push-chart
 	$(MAKE) gcloud-auth-docker push REGISTRY=$(release_registry) VERSION=$(RELEASE_VERSION)
 	git push -u upstream release-$(RELEASE_VERSION)