Bump Dockerfile to Alpine 3.8.1
Bump Dockerfile to Alpine 3.8.1

Notably, this patches the `apk` remote code execution vulnerability
(https://justi.cz/security/2018/09/13/alpine-apk-rce.html).

e14n/pump.io,e14n/pump.io,pump-io/pump.io,pump-io/pump.io,e14n/pump.io,pump-io/pump.io