Build flow for alpine from scratch

[mightyguava]

Kind of hacky prototype to get flow-bin to also publish a binary that works on alpine, which is a Linux distro quite a lot of people to run node in docker. I couldn't think of a better way than to build it within an alpine docker image and copy binary out. Probably would be better done in https://github.com/facebook/flow but the deploy system there seems to be entirely automated in Travis, which has stated that they will probably never support alpine.

So anyways, this will work as long as you have docker installed. Takes quite a bit of time to build. The check for alpine vs debian I stole from sass/node-sass#1589.

The alpine flow binary very much could be its own project and npm package, but it'd be great if yarn run flow could work within an alpine docker container just by installing flow-bin (well, and libelf...).

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