docker: Add support for buildtests inside Docker.

Makefile.buildtests

@@ -31,6 +31,9 @@ endif
         info-buildsizes-diff info-build info-boards-supported \
         info-features-missing info-boards-features-missing
 
+ifeq ($(BUILD_IN_DOCKER),1)
+buildtest: ..in-docker-container
+else
 buildtest:
 	@ \
 	BUILDTESTOK=true; \
@@ -66,6 +69,7 @@ buildtest:
 		done; \
 	done; \
 	$${BUILDTESTOK}
+endif # BUILD_IN_DOCKER
 
 info-objsize:
 	@case "${SORTROW}" in \

Makefile.docker

@@ -4,6 +4,7 @@ export DOCKER_FLAGS ?= --rm
 # List of Docker-enabled make goals
 export DOCKER_MAKECMDGOALS_POSSIBLE = \
   all \
+  buildtest \
   #
 export DOCKER_MAKECMDGOALS = $(filter $(MAKECMDGOALS),$(DOCKER_MAKECMDGOALS_POSSIBLE))