Required fix on docker-without-sudo

[pintify]

I'm working with Ubuntu 20 and I required to run also newgrp docker prior to see the groups configuration applied.

[sindresorhus]

Pull request welcome if someone does a (much!) better job than the previous low-quality pull requests:

• https://github.com/sindresorhus/guides/pull/58
• https://github.com/sindresorhus/guides/pull/59
• https://github.com/sindresorhus/guides/pull/49
• https://github.com/sindresorhus/guides/pull/51