tests/docker: set HOST_ARCH if we don't have ARCH

As the docker rules want to be able to be run on a virgin unconfigured
checkout add a fallback and use it if we need to.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>

.shippable.yml

@@ -27,8 +27,6 @@ env:
       TARGET_LIST=ppc64-softmmu,ppc64-linux-user,ppc64abi32-linux-user
 build:
   pre_ci:
-    # usually host ARCH is set by configure
-    - echo "ARCH=$(uname -m)" > config-host.mak
     - make docker-image-${IMAGE} V=1
   pre_ci_boot:
     image_name: qemu

tests/docker/Makefile.include

@@ -2,6 +2,8 @@
 
 .PHONY: docker docker-test docker-clean docker-image docker-qemu-src
 
+HOST_ARCH = $(if $(ARCH),$(ARCH),$(shell uname -m))
+
 DOCKER_SUFFIX := .docker
 DOCKER_FILES_DIR := $(SRC_PATH)/tests/docker/dockerfiles
 # we don't run tests on intermediate images (used as base by another image)
@@ -88,7 +90,7 @@ endif
 
 # Enforce dependencies for composite images
 docker-image-debian9-mxe: docker-image-debian9
-ifeq ($(ARCH),x86_64)
+ifeq ($(HOST_ARCH),x86_64)
 docker-image-debian-amd64: docker-image-debian9
 DOCKER_PARTIAL_IMAGES += debian-amd64-cross
 else
@@ -106,7 +108,7 @@ docker-image-debian-win32-cross: docker-image-debian9-mxe
 docker-image-debian-win64-cross: docker-image-debian9-mxe
 
 # For non-x86 hosts not all cross-compilers have been packaged
-ifneq ($(ARCH),x86_64)
+ifneq ($(HOST_ARCH),x86_64)
 DOCKER_PARTIAL_IMAGES += debian-mips-cross debian-mipsel-cross debian-mips64el-cross
 DOCKER_PARTIAL_IMAGES += debian-ppc64el-cross
 DOCKER_PARTIAL_IMAGES += debian-s390x-cross