makefiles/docker.inc.mk: handle building in git worktree

When building from a worktree, the common git directory was not mounted
in docker.

This lead to the version not being set and issues with git-cache in
ubuntu bionic that could not execute the 'git hash-object' command.

makefiles/docker.inc.mk

@@ -89,6 +89,11 @@ DOCKER ?= docker
 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)
 
+# Handle worktree by mounting the git common dir in the same location
+_is_git_worktree = $(shell grep '^gitdir: ' $(RIOTBASE)/.git 2>/dev/null)
+GIT_WORKTREE_COMMONDIR = $(shell git rev-parse --git-common-dir)
+DOCKER_VOLUMES_AND_ENV += $(if $(_is_git_worktree),-v $(GIT_WORKTREE_COMMONDIR):$(GIT_WORKTREE_COMMONDIR))
+
 # 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