In this case, I don’t believe it is the symlink that causes your issue. If you read https://beta.docker.com/docs/mac/osxfs/ in the section Namespaces, you will find that /Users, /Volumes, /tmp, and /private are the only directly shared paths. If you wish to use another path from the host, you need to prefix it with /Mac. This /Mac prefix and the behavior of host mounts will be changing soon and definitely before Docker for Mac leaves beta so please do no rely on it or keep careful record of where you use it so that you can easily change it later.
Hope this helps,
David