title: Pass ENV to Docker container running single command
body: <p>This prints blank:</p>
  
  <pre><code>docker run --rm --env HELLO="world" ubuntu:18.04 bash -c "echo $HELLO"
  </code></pre>
  
  <p>However this works:</p>
  
  <pre><code>docker run --rm -it --env HELLO="world" ubuntu:18.04 bash
  # in the container
  echo $HELLO
  </code></pre>
  
  <p><code>HELLO</code> seems passed to the container though:</p>
  
  <pre><code>docker run --rm --env HELLO="world" ubuntu:18.04 env
  </code></pre>
  
  <p>Why is the first command not seeing <code>HELLO</code>? What am I missing?</p>