tests/docker: add debian-amd64-cross for non-x86 hosts

When building on a non-x86 host we need to setup the x86 build like
any other cross compiler.

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

.shippable.yml

@@ -27,6 +27,8 @@ 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

@@ -89,7 +89,13 @@ endif
 
 # Enforce dependencies for composite images
 docker-image-debian9-mxe: docker-image-debian9
+ifeq ($(ARCH),x86_64)
 docker-image-debian-amd64: docker-image-debian9
+DOCKER_PARTIAL_IMAGES += debian-amd64-cross
+else
+docker-image-debian-amd64-cross: docker-image-debian10
+DOCKER_PARTIAL_IMAGES += debian-amd64
+endif
 docker-image-debian-armel-cross: docker-image-debian9
 docker-image-debian-armhf-cross: docker-image-debian9
 docker-image-debian-mips-cross: docker-image-debian9

tests/docker/dockerfiles/debian-amd64-cross.docker

@@ -0,0 +1,22 @@
+#
+# Docker x86_64 cross target
+#
+# This docker target is used on non-x86_64 machines which need the
+# x86_64 cross compilers installed.
+#
+FROM qemu:debian10
+MAINTAINER Alex Bennée <alex.bennee@linaro.org>
+
+# Add the foreign architecture we want and install dependencies
+RUN dpkg --add-architecture amd64
+RUN apt update && \
+    DEBIAN_FRONTEND=noninteractive eatmydata \
+    apt install -y --no-install-recommends \
+        crossbuild-essential-amd64
+RUN apt update && \
+    DEBIAN_FRONTEND=noninteractive eatmydata \
+    apt build-dep -yy -a amd64 --arch-only qemu
+
+# Specify the cross prefix for this image (see tests/docker/common.rc)
+ENV QEMU_CONFIGURE_OPTS --cross-prefix=x86_64-linux-gnu-
+ENV DEF_TARGET_LIST x86_64-softmmu,x86_64-linux-user,i386-softmmu,i386-linux-user