Add a ddev post-start hook to remove Dockerfile.playwright

[deviantintegral]

Once the containers are started, we don't need this file anymore. If we remove it, then we know that users won't accidentally commit it to git in case they miss that setup step.

[penyaskito]

tbh I'd prefer we add a gitignore file? Problem is that another add-on might be doing that too, and they would collide.

Maybe adding a .gitignore.ddev-playwright and calling in post-install

git config core.excludesfile .gitignore.ddev-playwright

is a better option.