add note about DOCKER_HOST for kn quickstart kind

[psschwei]

Fixes knative-extensions/kn-plugin-quickstart#617

Proposed Changes

Adds a snippet about setting DOCKER_HOST if running kn quickstart kind against a non-standard docker endpoint.

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[psschwei]

looks like this got rate limited

9:22:01 PM:   File "/opt/buildhome/.local/share/mise/installs/python/3.13.13/lib/python3.13/site-packages/github/Requester.py", line 378, in __check
9:22:01 PM:     raise self.__createException(status, responseHeaders, output)
9:22:01 PM: github.GithubException.RateLimitExceededException: 403 {"message": "API rate limit exceeded for 3.21.168.160. (But here's the good news: Authenticated requests get a higher rate limit. Check out the documentation for more details.)", "documentation_url": "https://docs.github.com/rest/overview/resources-in-the-rest-api#rate-limiting"}