Makefile: fix 'make shell'

Makefile

@@ -47,8 +47,8 @@ update-tools:
 	./src/make/update.sh
 
 .PHONY: shell
-shell: out/tools-image.digest
-	$(call run-container, -v $${PWD}:/home/user/stack:rw, $(shell cat $<), /bin/bash)
+shell: build-tools load-tools
+	$(call run-container, -v $${PWD}:/home/user/stack:rw, $(REGISTRY)/tools:latest, /bin/bash)
 
 .PHONY: credentials
 credentials: \
@@ -147,6 +147,9 @@ build-%: images/tools/Containerfile | out
 	cd images/tools
 	$(call build-container,$*,$(VERSION),$<,$(SOURCE_DATE_EPOCH),$(REVISION))
 
+load-%: build-%
+	$(call import-container,$*)
+
 out/tools-image.digest: out build-tools
 
 .PHONY: plan

src/make/macros.mk

@@ -48,3 +48,7 @@ define build-container
 		$(dir $3) \
 		| tar -C out/image/$(1) -mx
 endef
+
+define import-container
+	tar -C out/image/$(1) -cf - . | docker load
+endef