tests/docker: don't set DOCKER_REGISTRY on non-x86_64

Currently our gitlab registry is x86_64 only so attempting to pull an
image from it on something else will end in tears.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20210401102530.12030-3-alex.bennee@linaro.org>

tests/docker/Makefile.include

@@ -16,7 +16,10 @@ DOCKER_IMAGES := $(sort $(notdir $(basename $(wildcard $(DOCKER_FILES_DIR)/*.doc
 DOCKER_TARGETS := $(patsubst %,docker-image-%,$(DOCKER_IMAGES))
 # Use a global constant ccache directory to speed up repetitive builds
 DOCKER_CCACHE_DIR := $$HOME/.cache/qemu-docker-ccache
-DOCKER_REGISTRY := $(if $(REGISTRY),$(REGISTRY),registry.gitlab.com/qemu-project/qemu)
+ifeq ($(HOST_ARCH),x86_64)
+DOCKER_DEFAULT_REGISTRY := registry.gitlab.com/qemu-project/qemu
+endif
+DOCKER_REGISTRY := $(if $(REGISTRY),$(REGISTRY),$(DOCKER_DEFAULT_REGISTRY))
 
 DOCKER_TESTS := $(notdir $(shell \
 	find $(SRC_PATH)/tests/docker/ -name 'test-*' -type f))