docs: rework "Firefox not found" known problem

[MikeMcC399]

Issue

The README > Firefox not found description in the Common problems section is inaccurate.

Firefox can run under the root user. The problem only occurs in GitHub Actions when the user is root and the HOME environment variable is set to a non-root user's home directory.

Change

• The README > Firefox not found section is re-written and made independent of the cypress-io/github-action.

• The section is also linked to the open Cypress issue https://github.com/cypress-io/cypress/issues/27121.

[cypress-app-bot]

• Create a Draft Pull Request if your PR is not ready for review. Mark the PR as Ready for Review when you're ready for a Cypress team member to review the PR.

[MikeMcC399]

Preview

https://github.com/MikeMcC399/cypress-docker-images/blob/firefox-not-found/README.md#firefox-not-found

[MikeMcC399]

@jennifer-shehane

Thanks. This reads much better.

Thanks to getting involved more with Docker I finally understood the mechanisms and so I was able to explain it properly!