Update SSH tunneling docs to use Docker context

[karolz-ms]

After 1.1.0 with Docker context support ships, we should update SSH tunneling documentation and advise users to use Docker context instead of setting DOCKER_HOST environment variable explicitly.

With Docker context the user can switch between various Docker engines quickly and apply all configuration settings consistently.

[bwateratmsft]

Please note in the docs that an SSH agent is still needed to do SSH tunneling successfully