docker: Use system GIT_CACHE_DIR if available

If GIT_CACHE_DIR is a directory make it available to docker.
This will allow using the system git_cache also in the docker container.

makefiles/docker.inc.mk

@@ -83,6 +83,12 @@ DOCKER_OVERRIDE_CMDLINE := $(strip $(DOCKER_OVERRIDE_CMDLINE))
 # Overwrite if you want to use `docker` with sudo
 DOCKER ?= docker
 
+# Mounted volumes and exported environment variables
+
+# Add GIT_CACHE_DIR if the directory exists
+DOCKER_VOLUMES_AND_ENV += $(if $(wildcard $(GIT_CACHE_DIR)),-v $(GIT_CACHE_DIR):$(DOCKER_BUILD_ROOT)/gitcache)
+DOCKER_VOLUMES_AND_ENV += $(if $(wildcard $(GIT_CACHE_DIR)),-e GIT_CACHE_DIR=$(DOCKER_BUILD_ROOT)/gitcache)
+
 # This will execute `make $(DOCKER_MAKECMDGOALS)` inside a Docker container.
 # We do not push the regular $(MAKECMDGOALS) to the container's make command in
 # order to only perform building inside the container and defer executing any
@@ -105,6 +111,7 @@ DOCKER ?= docker
 	    -e 'RIOTBOARD=$(DOCKER_BUILD_ROOT)/riotboard' \
 	    -e 'RIOTMAKE=$(DOCKER_BUILD_ROOT)/riotmake' \
 	    -e 'RIOTPROJECT=$(DOCKER_BUILD_ROOT)/riotproject' \
+	    $(DOCKER_VOLUMES_AND_ENV) \
 	    $(DOCKER_ENVIRONMENT_CMDLINE) \
 	    -w '$(DOCKER_BUILD_ROOT)/riotproject/$(BUILDRELPATH)' \
 	    '$(DOCKER_IMAGE)' make $(DOCKER_MAKECMDGOALS) $(DOCKER_OVERRIDE_CMDLINE)