title: invalid argument "gcr.io//hello-app:v1" for "-t, --tag" flag: invalid reference format
body: <p>I'm following this tutorial: <a href="https://cloud.google.com/kubernetes-engine/docs/tutorials/hello-app" rel="nofollow noreferrer">https://cloud.google.com/kubernetes-engine/docs/tutorials/hello-app</a> for Google Cloud Platform. I'm using the Google Cloud Shell command line. When I got to the step:
  To build the container image of this application and tag it for uploading, run the following command:</p>
  
  <pre><code>docker build -t gcr.io/${PROJECT_ID}/hello-app:v1 .
  </code></pre>
  
  <p>I get an error:</p>
  
  <pre><code>invalid argument "gcr.io//hello-app:v1" for "-t, --tag" flag: invalid reference format
  </code></pre>
  
  <p>Bear in mind I already have 3 instances cluster (created from Kubernetes Engine) and one VM instance created on its own, existing in my VM instances, created from previous tutorials. Not sure if this has anything to do with the error.
  Thanks in advance.</p>