Feature Request: Add CI test for docker-entrypoint.sh

**Describe the Feature Request**

Add CI testing for the Xvfb script in `docker-entrypoint.sh`.

**Is your feature request related to a particular problem?**

The CI config [.circleci/config.yml](https://github.com/CircleCI-Public/cimg-node/blob/main/.circleci/config.yml) does not test the functionality of the `docker-entrypoint.sh` script used in `cimg/node:*-browsers` images. It is possible for PRs making changes to this script to pass CI tests and cause downstream issues. For example PR https://github.com/CircleCI-Public/cimg-node/pull/499 caused the essential `DISPLAY` environment variable to be removed. If this had been included in any released image it would have caused hard failures in Cypress pipelines using new images.

**How will this feature request benefit CircleCI jobs using this image?**

It will help prevent the release of images with bugs in the `docker-entrypoint.sh` script

**Describe the solution you would like to see**

Build an image and test the script `docker-entrypoint.sh` in CI when PRs are submitted and commits are merged into the default branch.

**Describe alternatives you have considered**

The only current alternative is to manually build a Docker image from a PR branch and then manually test it.