docs: Change the Alpine Docker example to use env instead of launch option

[neet]

This PR changes the documentation for the way to specify executablePath on Alpine Docker.

Formerly, the doc used options of puppeteer.launch, but since Puppeteer has an environment variable for it, I think it would be better to use it instead of specifying it every time.

I've already verified this can be built through the latest version of Docker. The Docker image can be found at DockerHub and the source code it at neetbox/pptr-alpine-docker.

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[neet]

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