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.
Not all users register their user in the docker group and should not run the
whole make process as root to use docker.
They can now overwrite DOCKER with `DOCKER="sudo docker"`.
Creating files as a user is correctly handled as `id -u` is still run as the
original user.
