Consider using docker image for automated tests in Github Actions

[jf-x-dev]

We currently cache/install Playwright + browsers. We should technically also install the Playwright dependencies every time before running tests, but:

• Installing these can hang, taking a long time
• For Chrome and Firefox tests, this is not strictly necessary (it does show a large "Host system is missing dependencies to run browsers." warning in the console though 😅)

So currently we just skip this step (unless we are emulating tests on an iOS device). It would be better to retrieve a docker image with playwright, browsers and dependencies pre-installed and run tests from within it.