title: Adding GPG key inside docker container causes "no valid OpenPGP data found"
body: <p>I'm trying to install New Relic's system monitoring inside a docker container, but the <code>apt-key add -</code> fails with <code>no valid OpenPGP data found</code>.</p>
  
  <p>There is the full <code>Dockerfile</code>:</p>
  
  <pre><code>FROM ubuntu
  MAINTAINER Matej Koubik
  
  RUN echo deb http://apt.newrelic.com/debian/ newrelic non-free &gt;&gt; /etc/apt/sources.list.d/newrelic.list
  RUN wget -O- https://download.newrelic.com/548C16BF.gpg | apt-key add -
  RUN apt-get update
  RUN apt-get install newrelic-sysmond
  RUN nrsysmond-config --set license_key=...
  RUN /etc/init.d/newrelic-sysmond start
  </code></pre>