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.

Mounting `/etc/localtime` directly does not work on macOS (anymore).
However, by resolving the symlink to its real path docker can handle
the mount.

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.

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.

The variable was not given to docker before and as it is setting
'RIOT_VERSION_OVERRIDE=buildtest' it helps testing in docker more
consistently.

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.

prevents "the input device is not a TTY"

Can be overridden from the environment by setting WERROR=0

also remove reversed logic for Werror in Makefile.buildtest
(a typo caused Werror to be always enabled for all builds)

 - Better support for passing environment variables to the Docker container.
 - Move Docker-related definitions into Makefile.docker.
 - Only environment variables that originates from the command line or
   the environment will be passed to the Docker container.