Docker task should allow variables to be used in the docker image/repository name

The Docker task does not allow using variables to point to the name of the image to be used for the task.

The repository field in the "Build a Docker Image" and "Push a Docker image to a Docker registry" commands prevents the ${variable} syntax from being entered, as it does not meet the regex requirements for a Docker repository name.

The Docker image field in the "Run a Docker container" command does allow the ${variable} syntax, but does not substitute the variable when attempting to run the container, and the Docker daemon returns an error.