[BUG] more user-friendliness when using docker containers

[aslushnikov]

Based on the feedback we received, here are two action items to improve user experience of playwright:

• Add pwuser back to the playwright docker image, but keep running containers under root. Motivation: This would be similar to the node user in the node docker images, and nginx user in nginx images.
• Switch chromiumSandbox to false by default. Motivation: Certain environments like Azure Pipelines override default container user with a custom one and fail to launch container with a proper seccomp profile. Auto-detecting chromium sandboxing capabilities is hard - the best way would be to launch a browser and wait for renderer crash. This is a fragile approach; instead, default to no sandbox similarly to the webkit.

[aslushnikov]

cc'ing @mxschmitt who had a patch for pwuser.

#3899