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...).

[facebook-github-bot]

Thank you for your pull request and welcome to our community. We require contributors to sign our Contributor License Agreement, and we don't seem to have you on file. In order for us to review and merge your code, please sign up at https://code.facebook.com/cla - and if you have received this in error or have any questions, please drop us a line at cla@fb.com. Thanks!

If you are contributing on behalf of someone else (eg your employer): the individual CLA is not sufficient - use https://developers.facebook.com/opensource/cla?type=company instead. Contact cla@fb.com if you have any questions.

[facebook-github-bot]

Thank you for signing our Contributor License Agreement. We can now accept your code for this (and any) Facebook open source project. Thanks!