[FEAT] - Refactor Dockerfile

[dgrebb]

Ensure proper configuration set for

• [ ] user: does it need to be root? node? something else?
• [ ] deps: npx playwright install --with-deps in the Dockerfile itself to reduce build times
• [ ] speed: set up container layering so all dependencies are cached prior to setting up backstop in the RUN command
• [ ] any other nice-to haves from open issues