title: E: Unable to locate package usbmount
body: <p>I've installed a debian stretch container in docker (via portainer.io) 
  and was able to install openssh-server via apt-get install.</p>
  
  <p>After a successful connection with PuTTY, i'm not able to install usbmount which i need for later purpose..</p>
  
  <p>i searched a lot and went through different repositories but nothing helped - i'm getting the same error every time. </p>
  
  <p>of course, i <strong>typed in apt-get update after every edit of sources.list</strong></p>
  
  <p>by looking for another solution i've tried to install apt-file and for some reason this works, but usbmount wont...</p>
  
  <p>this is the content of my /etc/apt/sources.list</p>
  
  <pre><code>deb http://deb.debian.org/debian/ stable main contrib non-free
  deb-src http://deb.debian.org/debian/ stable main contrib non-free
  
  deb http://deb.debian.org/debian/ stable-updates main contrib non-free
  deb-src http://deb.debian.org/debian/ stable-updates main contrib non-free
  
  deb http://deb.debian.org/debian-security stable/updates main
  deb-src http://deb.debian.org/debian-security stable/updates main
  
  deb http://ftp.debian.org/debian stretch-backports main
  deb-src http://ftp.debian.org/debian stretch-backports main
  </code></pre>
  
  <p>any suggestions?
  thanks in advance</p>