The 'build' directory should be created before starting docker.
If not it will be created as root.
Also add mapping for the directory in docker.
Currently create the directory in the target until there is a directory
creation target.
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.
