As larsks mentions, apk is for Alpine distributions and you selected FROM ubuntu:trusty which is Debian based with the apt-get command. Change your FROM line to FROM alpine:3.4 to switch to the Alpine based image with apk support.
As larsks mentions, apk is for Alpine distributions and you selected FROM ubuntu:trusty which is Debian based with the apt-get command. Change your FROM line to FROM alpine:3.4 to switch to the Alpine based image with apk support.