title: What are <none> repository and tags? Why do they appear when I use docker build?
body: <p>This is what <code>docker images</code> shows before I run <code>docker build</code>.</p>
  
  <pre><code>$ docker images
  REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
  myapp               latest              a38da0bd9e0b        6 seconds ago       523.8 MB
  golang              onbuild             b4997c557048        10 days ago         517.2 MB
  </code></pre>
  
  <p>After I make some changes to the <code>myapp</code> source code, I run <code>docker build -t myapp .</code> and I end up with images named <code>&lt;none&gt;</code>.</p>
  
  <pre><code>$ docker images
  REPOSITORY          TAG                 IMAGE ID            CREATED                      VIRTUAL SIZE
  myapp               latest              a38da0bd9e0b        Less than a second ago ago   523.8 MB
  &lt;none&gt;              &lt;none&gt;              e4209f97e819        10 minutes ago               523.8 MB
  golang              onbuild             b4997c557048        10 days ago                  517.2 MB
  </code></pre>
  
  <p>I know I can remove them with <code>docker rmi &lt;IMAGE ID&gt;</code>, but why does that happen in the first place? How can I prevent that from happening? The <code>Dockerfile</code> I'm building looks like this.</p>
  
  <pre><code>FROM golang:onbuild
  EXPOSE 8080
  </code></pre>