title: Docker: `Repository name must match ...` error
body: <p>I'm reading the book <code>Docker in action</code>, which is a really great book so far, but I think I'm stuck now on a command which doesn't work</p>
  
  <pre><code>$&gt; docker run –it --rm --link cass1:cass cassandra:2.2 cqlsh cass
  </code></pre>
  
  <p>It should run an interactive shell (<code>cqlsh</code>) on the cassandra database, but when I run this I get the following error: </p>
  
  <pre><code>repository name component must match "[a-z0-9](?:-*[a-z0-9])*(?:[._][a-z0-9](?:-*[a-z0-9])*)*"
  </code></pre>
  
  <p>Any suggestions why this doesn't work ?</p>