title: SBT docker:publish authentication issue
body: <p>I am Using SBT Native Packager to build <strong>Docker</strong> Image,
  and my <em>build.sbt</em> is :</p>
  
  <pre><code>   packageName in Docker := name.value
     version in Docker := version.value
     dockerBaseImage := "java"
     dockerRepository :=Some("index.docker.io/xyz")
     dockerExposedPorts := Seq(8283)
  </code></pre>
  
  <p>I am trying to publish image to private docker hub,but its giving </p>
  
  <pre><code>[error] unauthorized: authentication required.
  </code></pre>
  
  <p>in build.sbt how can i specify hub <strong><em>userName</em></strong> and <strong><em>Password</em></strong>.</p>
  
  <p>is there any other configuration that i am missing here. </p>