title: DOCKER_REGISTRY value is not set build error
body: <p>I'm doing some experimenting with .NET Core 2.1, React/Redux, and Docker.  Without making any changes to the boilerplate ASP.NET Core with React code generated, I added Docker support to the project.</p>
  
  <p>Now the project doesn't build!</p>
  
  <p>When I attempt to build, I get the error 
  <code>DOCKER_REGISTRY value is not set build error.</code></p>
  
  <p>Attempting to fix this, I tried changing my docker-compose file to be like this:</p>
  
  <pre><code>version: '3.4'
  
  services:
    project:
      image: ${DOCKER_REGISTRY}project
      build:
        context: .
        dockerfile: Project\Dockerfile
      environment:
        DOCKER_REGISTRY = "docker.io"
  </code></pre>
  
  <p>But, this didn't work. I assume I'm missing something pretty straightforward.</p>
  
  <p>Thanks in advance!</p>
  
  <p>UPDATE:</p>
  
  <p>I got a little bit further by thinking that I don't have an image saved in docker.io, so why should I be trying to pull one down? I changed my docker-compose file to look like this:</p>
  
  <pre><code>version: '3.4'
  
  services:
    project:
      build:
        context: .
        dockerfile: project\Dockerfile
  </code></pre>
  
  <p>And now the project will at least build - but it won't debug, because I now get an error on the second step of 7:</p>
  
  <pre><code>Service 'project' failed to build: COPY failed: CreateFile \\?\C:\ProgramData\Docker\tmp\docker-builder965605424\src: The system cannot find the file specified..
  </code></pre>