[Prebuilds] Allow use of docker during prebuilds

[svenefftinge]

In order to use docker during a prebuild, we need to run docker-up which is a non terminating process.
Would be good to come up with a simple solution instead of having everyone hacking this by manually stopping the docker service when done.

[svenefftinge]

We can generalize this to adding some sort of explicit shutdown hook, that tells gitpod the prebuild is finished.

[csweichel]

Alternatively, we could provide a "background" task type that gets run before everything else, and gets stopped just before the workspace shuts down (after a potential future teardown hook).

[weitzman]

Looks like #2654 may have made progress here but status is not clear to me.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.