Update docker-build steps in Makefile.release (#1200)

* Update docker-build steps in Makefile.release

for different architectures
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Update to use `case ... esac` in shell script
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

* Remove `RUN` cmd in Dockerfile in case of non amd64 CPU

ssl will only be supported on amd64 CPUs for now.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

Makefile.release

@@ -100,9 +100,35 @@ upload:
 
 .PHONY: docker-build
 docker-build: tar
+	# Steps:
+	# 1. Copy appropriate coredns binary to build/docker/linux/<arch>
+	# 2. Copy Dockerfile to build/docker/linux/<arch>
+	# 3. Replace base image from alpine:latest to <arch>/alpine:latest
+	# 4. Comment RUN in Dockerfile
+	# <arch>:
+	# arm: arm32v6
+	# arm64: arm64v8
+	rm -rf build/docker
 	for arch in $(LINUX_ARCH); do \
-	    tar -xzf release/$(NAME)_$(VERSION)_linux_$$arch.tgz ;\
-	    docker build -t coredns . ;\
+	    mkdir -p build/docker/linux/$$arch ;\
+	    tar -xzf release/$(NAME)_$(VERSION)_linux_$$arch.tgz -C build/docker/linux/$$arch ;\
+	    cp Dockerfile build/docker/linux/$$arch ;\
+	    if [ $$arch != amd64 ]; then \
+		case $$arch in \
+	        arm) \
+	            BASEIMAGE=arm32v6 ;\
+	            ;; \
+	        arm64) \
+	            BASEIMAGE=arm64v8 ;\
+	            ;; \
+	        *) \
+	            BASEIMAGE=$$arch ;\
+	            ;; \
+	        esac ;\
+	        sed -e "s/alpine:latest/$$BASEIMAGE\\/alpine:latest/" -i build/docker/linux/$$arch/Dockerfile ;\
+	        sed -e "s/^\\s*RUN/#RUN/" -i build/docker/linux/$$arch/Dockerfile ;\
+	    fi ;\
+	    docker build -t coredns build/docker/linux/$$arch ;\
 	    docker tag coredns $(DOCKER_IMAGE_NAME):coredns-$$arch ;\
 	done