title: Docker build fails on `add-apt-repository: not found`
body: <p>My docker build fails on:</p>
  
  <pre><code> ---&gt; Running in a0d2edd6bf20
  /bin/sh: 1: add-apt-repository: not found    
  The command '/bin/sh -c add-apt-repository ppa:jonathonf/ffmpeg-3 &amp;&amp; apt-get update &amp;&amp; apt install -y libavformat-dev libavcodec-dev libavdevice-dev libavutil-dev libswscale-dev libavresample-dev ffmpeg libav-tools x264 x265' returned a non-zero code: 127
  </code></pre>
  
  <p>I've researched the problem and it says to install either <code>software-properties-common</code> or <code>python-software-properties</code> </p>
  
  <p>I did just that but the error still persists. Here is my Dockerfile up until that point</p>
  
  <pre><code>FROM ubuntu:16.04
  
  ENV HOME_DIR=/root
  
  WORKDIR $HOME_DIR
  
  RUN apt-get update &amp;&amp; apt-get install -y \
      pkg-config \
      git \
      cmake \
      build-essential \
      nasm \
      wget \
      python3-setuptools \
      libusb-1.0-0-dev  \
      python3-dev \
      python3-pip \
      python3-numpy \
      python3-scipy \
      libglew-dev \
      libglfw3-dev \
      libtbb-dev \
      software-properties-common \
      python-software-properties 
  
  RUN add-apt-repository ppa:jonathonf/ffmpeg-3 &amp;&amp; \
      apt-get update &amp;&amp; \
      apt install -y \
      libavformat-dev \
      libavcodec-dev \
      libavdevice-dev \
      libavutil-dev \
      libswscale-dev \
      libavresample-dev \
      ffmpeg \
      libav-tools \
      x264 \
      x265
  </code></pre>