tests/docker: fix the IMAGE for build invocation

We inadvertently broke the ability to run local builds when the code
was re-factored. The result was the run stanza failing to find the
docker image with it's qemu/ prefix.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Fixes: d39eaa22 ("tests/docker: simplify docker-TEST@IMAGE targets")
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20220613171258.1905715-5-alex.bennee@linaro.org>

tests/docker/Makefile.include

@@ -171,7 +171,7 @@ DOCKER_TESTS := $(if $(TESTS), $(filter $(TESTS), $(__TESTS)), $(__TESTS))
 $(foreach i,$(filter-out $(DOCKER_PARTIAL_IMAGES),$(DOCKER_IMAGES)), \
 	$(foreach t,$(DOCKER_TESTS), \
 		$(eval .PHONY: docker-$t@$i) \
-		$(eval docker-$t@$i: docker-image-$i; @$(MAKE) docker-run TEST=$t IMAGE=$i) \
+		$(eval docker-$t@$i: docker-image-$i; @$(MAKE) docker-run TEST=$t IMAGE=qemu/$i) \
 	) \
 	$(foreach t,$(DOCKER_TESTS), \
 		$(eval docker-all-tests: docker-$t@$i) \