apt-add-repository in Dockerfile

This is an alternative to apt-add-repository which is useful for building docker images. The following is an example for adding Phalcon repository from launchpad.net

FROM ubuntu
MAINTAINER maintainer@example.com

RUN apt-key adv --keyserver hkp://keyserver.ubuntu.com:80 --recv 1E569794
RUN echo "deb http://ppa.launchpad.net/phalcon/stable/ubuntu trusty main" > /etc/apt/sources.list.d/phalcon-stable-trusty.list

See the picture below for where magic values are from. But I have no idea where “/etc/apt/sources.list.d/phalcon-stable-trusty.list” is from though.