Improve instructions for installing the required `bashbrew`

[jstirnaman]

Neither the docker-library README nor the bashbrew README are clear in explaining how to install bashbrew (required for rebuilding image docs).

[tianon]

Oh, good callout -- if you just need a quick fix, you can find binaries at https://github.com/docker-library/bashbrew/releases

[tianon]

I've fixed/updated the link in 40d0748, as a starter 👍

[stevepiercy]

The docs also do not say what to do with a downloaded binary to install it.

[stevepiercy]

For Apple M2 Max chips:

1. Download bashbrew-darwin-amd64 from the latest releases.
2. Move to any location in your $PATH, such as /usr/local/bin.
3. chmod 755 /usr/local/bin/bashbrew-darwin-amd64.
4. ln -s /usr/local/bin/bashbrew-darwin-amd64 /usr/local/bin/bashbrew.
5. Follow https://support.apple.com/guide/mac-help/open-a-mac-app-from-an-unidentified-developer-mh40616/mac.
6. Run ./update.sh MY_IMAGE.