brew.sh: allow Docker to run as root.

[MikeMcQuaid]

Now I've used Docker more I accept this is more commonplace in that world.

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew tests with your changes locally?

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[MikeMcQuaid]

@sjackman Worth updating any of this with this in mind: https://github.com/Homebrew/brew/blob/master/Dockerfile#L35-L39?

[sjackman]

Absolutely! One concern is that there's likely Dockerfiles in the wild that inherit from this Docker image and expect there to be a linuxbrew user, and expect to be running as that user. I'm certain that I have a few myself. I'd suggest we still create the Linuxbrew user, but make the default user root. That way, fixing a downstream image just means adding USER linuxbrew.

[MikeMcQuaid]

I'd suggest we still create the Linuxbrew user, but make the default user root. That way, fixing a downstream image just means adding USER linuxbrew.

@sjackman Seems like a good idea 👍

Changes addressed.