tests/docker: add NOFETCH env variable for testing

Testing non-auto built docker containers (i.e. custom built compilers)
is a bit fiddly as you couldn't continue a build with a previously
locally built container. While you can play games with REGISTRY its
simpler to allow a NOFETCH that will go through the cached build
process when you run the tests.

Reviewed-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20241023113406.1284676-3-alex.bennee@linaro.org>

tests/docker/Makefile.include

@@ -92,10 +92,10 @@ endif
 docker-image-alpine: NOUSER=1
 
 debian-toolchain-run = \
-	$(if $(NOCACHE), 						\
+	$(if $(NOCACHE)$(NOFETCH),					\
 		$(call quiet-command,					\
 			$(DOCKER_SCRIPT) build -t qemu/$1 -f $< 	\
-			$(if $V,,--quiet) --no-cache 			\
+			$(if $V,,--quiet) $(if $(NOCACHE),--no-cache)	\
 			--registry $(DOCKER_REGISTRY) --extra-files	\
 			$(DOCKER_FILES_DIR)/$1.d/build-toolchain.sh,	\
 			"BUILD", $1),				        \
@@ -177,6 +177,7 @@ docker:
 	@echo '    NETWORK=$$BACKEND     Enable virtual network interface with $$BACKEND.'
 	@echo '    NOUSER=1             Define to disable adding current user to containers passwd.'
 	@echo '    NOCACHE=1            Ignore cache when build images.'
+	@echo '    NOFETCH=1            Do not fetch from the registry.'
 	@echo '    EXECUTABLE=<path>    Include executable in image.'
 	@echo '    EXTRA_FILES="<path> [... <path>]"'
 	@echo '                         Include extra files in image.'