Add info for DOCKER_HOST value in Using docker:dind

What does this MR do?

In Kubernetes executor page, in the Using dind section, the hostname in DOCKER_HOST value is localhost. For GitLab Runner versions greater than 12.7 and Kubernetes versions greater than 1.6, this is not correct; it should be 'docker' and not localhost.

This is described in the sample ci job script's comments in Docker-in-Docker with TLS enabled in Kubernetes:

DOCKER_HOST: tcp://docker:2376 #

# The 'docker' hostname is the alias of the service container as described at

# https://docs.gitlab.com/ee/ci/docker/using_docker_images.html#accessing-the-services.

# If you're using GitLab Runner 12.7 or earlier with the Kubernetes executor and Kubernetes 1.6 or earlier,

# the variable must be set to tcp://localhost:2376 because of how the

# Kubernetes executor connects services to the job container

# DOCKER_HOST: tcp://localhost:2376

Related issues

ZD ticket 229041

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