Based on the elixir alpine 1.6 official docker image (available on official repo).
This Dockerfile adds a Phoenix layer as well as the necessary dependencies to run on Wercker CI.
Wercker CI supports Alpine images, although it is necessary to explicitly specify the usage of sh instead of bash. This can be achieved by specifying the cmd property in the box section of your wercker.yml.
box:
id: rakutenbr/phoenix-alpine
cmd: /bin/sh
Further reference can be found at wercker faq.