Support cross-compile for arm

Pretty much cross-compile doesn't work because  of this:

> profiles/seccomp/seccomp.go:13:2: build constraints exclude all Go files in /go/src/github.com/docker/docker/vendor/github.com/seccomp/libseccomp-golang

This changes adds a new Dockerfile target for cross compilation with the
neccesary arch specific libseccomp packages and CC toolchains.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Dockerfile

@@ -24,6 +24,8 @@
 # the case. Therefore, you don't have to disable it anymore.
 #
 
+ARG CROSS="false"
+
 FROM golang:1.12.3 AS base
 # allow replacing httpredir or deb mirror
 ARG APT_MIRROR=deb.debian.org
@@ -93,11 +95,31 @@ RUN /download-frozen-image-v2.sh /build \
 # See also ensureFrozenImagesLinux() in "integration-cli/fixtures_linux_daemon_test.go" (which needs to be updated when adding images to this list)
 
 # Just a little hack so we don't have to install these deps twice, once for runc and once for dockerd
-FROM base AS runtime-dev
+FROM base AS runtime-dev-cross-false
 RUN apt-get update && apt-get install -y \
 	libapparmor-dev \
 	libseccomp-dev
 
+FROM runtime-dev-cross-false AS runtime-dev-cross-true
+RUN dpkg --add-architecture armhf
+RUN dpkg --add-architecture arm64
+RUN dpkg --add-architecture armel
+# These crossbuild packages rely on gcc-<arch>, but this doesn't want to install
+# on non-amd64 systems.
+# Additionally, the crossbuild-amd64 is currently only on debian:buster, so
+# other architectures cannnot crossbuild amd64.
+RUN if [ "$(go env GOHOSTARCH)" = "amd64" ]; then \
+	apt-get update \
+	&& apt-get install -y \
+		crossbuild-essential-armhf \
+		crossbuild-essential-arm64 \
+		crossbuild-essential-armel \
+		libseccomp-dev:armhf \
+		libseccomp-dev:arm64 \
+		libseccomp-dev:armel; \
+	fi
+
+FROM runtime-dev-cross-${CROSS} AS runtime-dev
 
 FROM base AS tomlv
 ENV INSTALL_BINARY_NAME=tomlv

Makefile

@@ -117,9 +117,6 @@ INTERACTIVE := $(shell [ -t 0 ] && echo 1 || echo 0)
 ifeq ($(INTERACTIVE), 1)
 	DOCKER_FLAGS += -t
 endif
-ifeq ($(BIND_DIR), .)
-	DOCKER_BUILD_OPTS += --target=dev
-endif
 
 DOCKER_RUN_DOCKER := $(DOCKER_FLAGS) "$(DOCKER_IMAGE)"
 
@@ -134,6 +131,21 @@ binary: build ## build the linux binaries
 dynbinary: build ## build the linux dynbinaries
 	$(DOCKER_RUN_DOCKER) hack/make.sh dynbinary
 
+
+
+cross: DOCKER_CROSS := true
+cross: build ## cross build the binaries for darwin, freebsd and\nwindows
+	$(DOCKER_RUN_DOCKER) hack/make.sh dynbinary binary cross
+
+ifdef DOCKER_CROSSPLATFORMS
+build: DOCKER_CROSS := true
+else
+build: DOCKER_CROSS ?= false
+endif
+ifeq ($(BIND_DIR), .)
+build: DOCKER_BUILD_OPTS += --target=dev
+endif
+build: DOCKER_BUILD_ARGS += --build-arg=CROSS=$(DOCKER_CROSS)
 build: DOCKER_BUILDKIT ?= 1
 build: bundles
 	$(warning The docker client CLI has moved to github.com/docker/cli. For a dev-test cycle involving the CLI, run:${\n} DOCKER_CLI_PATH=/host/path/to/cli/binary make shell ${\n} then change the cli and compile into a binary at the same location.${\n})
@@ -149,9 +161,6 @@ clean: clean-cache
 clean-cache:
 	docker volume rm -f docker-dev-cache
 
-cross: build ## cross build the binaries for darwin, freebsd and\nwindows
-	$(DOCKER_RUN_DOCKER) hack/make.sh dynbinary binary cross
-
 help: ## this help
 	@awk 'BEGIN {FS = ":.*?## "} /^[a-zA-Z0-9_-]+:.*?## / {gsub("\\\\n",sprintf("\n%22c",""), $$2);printf "\033[36m%-20s\033[0m %s\n", $$1, $$2}' $(MAKEFILE_LIST)
 

hack/make/.binary

@@ -47,6 +47,26 @@ if [ "$(go env GOOS)/$(go env GOARCH)" != "$(go env GOHOSTOS)/$(go env GOHOSTARC
 			export CC=x86_64-w64-mingw32-gcc
 			export CGO_ENABLED=1
 			;;
+		linux/arm)
+			case "${GOARM}" in
+			5|"")
+				export CC=arm-linux-gnueabi-gcc
+				export CGO_ENABLED=1
+				;;
+			7)
+				export CC=arm-linux-gnueabihf-gcc
+				export CGO_ENABLED=1
+				;;
+			esac
+			;;
+		linux/arm64)
+			export CC=aarch64-linux-gnu-gcc
+			export CGO_ENABLED=1
+			;;
+		linux/amd64)
+			export CC=x86_64-linux-gnu-gcc
+			export CGO_ENABLED=1
+			;;
 	esac
 fi