title: docker + apt-add-repository: 'ascii' codec can't decode byte
body: <p>After starting a fresh docker Ubuntu environment,</p>
  
  <pre><code>docker run -ti ubuntu:17.04
  </code></pre>
  
  <p>I'm trying to add a PPA via</p>
  
  <pre><code>apt-get update
  apt-get install -y software-properties-common
  apt-add-repository -y ppa:nschloe/vtk7-nightly
  </code></pre>
  
  <p>However, this fails with</p>
  
  <pre><code>gpg: key ECD154D280FEB8AC: public key "Launchpad PPA for Nico Schl�mer" imported
  gpg: Total number processed: 1
  gpg:               imported: 1
  'ascii' codec can't decode byte 0xc3 in position 229: ordinal not in range(128)
  </code></pre>
  
  <p>Looks like the dreaded <code>ö</code> umlaut strikes again.</p>
  
  <p>How to fix this?</p>