Build this as a multi-arch image

[djmaze]

Since the erlang base image is already multi-arch, it shouldn't be a problem building this for all supported architectures.

[c0b]

that's very right; but actually depends on who have those different architecture hardware, and can have a test? and the actual changes would be in docker-library/official-images repo instead; would @tianon be able to do same work as in erlang/docker-erlang-otp#70 for this list of arm32v7, arm64v8, i386, s390x, ppc64le I've just got an Rasberry Pi3 board will do some testing of the arm based ones

1. https://github.com/docker-library/official-images/blob/master/library/elixir
2. https://github.com/docker-library/official-images/blob/master/library/erlang