title: Docker invalid tag value
body: <p>I'm trying to build the OwnCloud desktop client, for which <a href="https://doc.owncloud.org/desktop/2.1/building.html#windows-installer-build-cross-compile" rel="noreferrer">the Owncloud docs</a> explain that docker should be used:</p>
  
  <pre><code>Assuming you are in the root of the ownCloud Client’s source tree, you
  can build an image from this Dockerfile like this:
  
  cd admin/win32/docker
  docker build . -t ownCloud-client-win32:&lt;version&gt;
  </code></pre>
  
  <p>I get an error with this exact command, so following <a href="https://docs.docker.com/engine/reference/commandline/build/" rel="noreferrer">the docker build docs</a> I moved the dot to the end. But then I get the following error:</p>
  
  <pre><code>$ docker build -t ownCloud-client-win32:2.1 .
  invalid value "ownCloud-client-win32:2.1" for flag -t: Error parsing reference: "ownCloud-client-win32:2.1" is not a valid repository/tag
  See 'docker build --help'.
  </code></pre>
  
  <p>Why is <code>ownCloud-client-win32:2.1</code> not a valid tag name?</p>
  
  <p>I then just tried running  <code>docker build .</code>, which runs successfully, but then I end up with an untagged image.</p>
  
  <p>Does anybody have any idea how I can solve this?</p>
  
  <p>ps. I don't know if it's important, but the relevant <a href="https://github.com/owncloud/client/blob/master/admin/win/docker/Dockerfile" rel="noreferrer">Dockerfile is here</a>.</p>