title: How do I start a Docker container with new environmental variables WITHOUT overwriting existing environmental variables?
body: <p>I'd like to pass environment variables into a new container WITHOUT overwriting existing environmental variables. I've tried using <code>--env</code> but unfortunately it overwrites everything. I also can't set the variables after the container is already started - the software I'm using would get mad.</p>
  <p>How can I send these variables in? I hope there's an option other than making a brand new image.</p>
  <p>Thank you so much for your help </p>
  <p>EDIT:</p>
  <p>The image I'm launching already has preset environmental variables. Those preset environmental variables are overwritten when I run the following code:</p>
  <p><code>docker run --interactive --tty --rm --name $name --volume $mount --workdir $workdir -p $p --env DISPLAY=docker.for.mac.host.internal:0 $image bash</code></p>
  <p>I want to set the environmental variable <code>DISPLAY=docker.for.mac.host.internal:0</code> when my container launches without overwriting the preset environmental variables.</p>
  <p>Sorry I confused everyone. I hope this is clear now!</p>