[WIP] Build docker image from ruby:2.3-alpine

[adammcmaster]

This changes the Dockerfile to build FROM ruby:2.3-alpine -- there's really no reason for us to build our own ruby image when there's an official one, and the -alpine version is tiny. This reduces the final panoptes image size to 130 MB (down from the current 248 MB).

This will need a thorough test when it's deployed to staging. Alpine uses musl libc rather than glibc, so we'll need to make sure this doesn't have any unintended effects.

[camallen]

can we move this to a staging docker file that we can use for the staging infrastructure via the dockerfile: Dockerfile.staging cmd? then we can get this out and running to see how it performs.

[adammcmaster]

After testing it looks like this will depend on rubyjs/therubyracer#378 unless we can find a workaround.

[camallen]

closing this for now - that linked issue is pretty stale. Let's re-open when we look at this again.