CI: consistently use DOCKER_CMD in makefiles

This allows rebuilding containers using podman too.

Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>

automation/build/Makefile

@@ -31,8 +31,8 @@ clean:
 define CLEAN_RULE
 .PHONY: clean-$(1)
 clean-$(1):
-	if [ -n "$$$$(docker image ls -q $(REGISTRY)/$(subst /,:,$(1)))" ]; then \
-		docker image rm $(REGISTRY)/$(subst /,:,$(1)); \
+	if [ -n "$$$$($(DOCKER_CMD) image ls -q $(REGISTRY)/$(subst /,:,$(1)))" ]; then \
+		$(DOCKER_CMD) image rm $(REGISTRY)/$(subst /,:,$(1)); \
 	fi
 
 endef

automation/tests-artifacts/Makefile

@@ -10,9 +10,9 @@ help:
 	@echo "To push container builds, set the env var PUSH"
 
 %: %.dockerfile ## Builds containers
-	docker build --pull -t $(REGISTRY)/$(@D):$(@F) -f $< $(<D)
+	$(DOCKER_CMD) build --pull -t $(REGISTRY)/$(@D):$(@F) -f $< $(<D)
 	@if [ ! -z $${PUSH+x} ]; then \
-		docker push $(REGISTRY)/$(@D):$(@F); \
+		$(DOCKER_CMD) push $(REGISTRY)/$(@D):$(@F); \
 	fi
 
 .PHONY: all