Warning not to use build:args for secrets

[Binarytales]

Assuming that

build:
    args:
        - key=token

is analogous to --build-arg <varname>=<value> command line option and it's realtion opt the ARG command in a Dockerfile, should the documentation not include the same warning about not using this to pass secrets as seen in the Dockerfile documentation:

https://docs.docker.com/engine/reference/builder/#/arg

Related PR/Issue: moby/moby#24522

That the example uses password:secret seems particularly egregious if my assumption is correct.

If somebody can clarify my assumption in the affirmative I'll open a PR to update the examples and add the same wording as a warning from PR linked above.

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