core: Support running on MacOS.

We can run shrinkwrap on MacOS as long as docker desktop is installed.
There are a couple of issues to work around though:

- readlink on MacOS does not have the -f (follow) option. Its not really
  important, so just remove use of it entirely.
- We were getting an error when trying to add the GID to the container
  due to Linux already having allocated the GID. So just run as root
  within the container on MacOS. The MacOS version of docker appears to
  ensure that files on the host file system have the correct permissions
  anyway.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>