docker run --link is deprecated

[AkihiroSuda]

https://github.com/docker-library/docs/tree/44e1657a6d4c7ec1c4aa803d247d093a330d9cfc/docker

The dind content.md suggests using docker run --link, but it has been deprecated: https://docs.docker.com/network/links/

[wglambert]

A lot of the --link's were replaced in #1441

https://github.com/docker-library/faq#--link-is-deprecated

[tianon]

Indeed -- this one was left alone because it was still using the DOCKER_PORT_2375_TCP environment variable for detection, which is the reason why https://github.com/docker-library/docker/pull/166/files#diff-60713fb19f6c50a2473835df63ff17f9L15 included changes to always set DOCKER_HOST if it isn't set and /var/run/docker.sock doesn't exist (with the express goal of updating this documentation).

[tianon]

#1538 👍