Fix Docker permissions

[manics]

A couple of fixes to get the docker node running:

• Ensure the docker group id inside the container and outside the host are the same (needed since /var/run/docker.sock is bind-mounted
• The workspace is shared between the Jenkins node and the ephemeral container, so can't be transiently created

[joshmoore]

Is the docker now being used?

[manics]

Not in the new jobs, though apparently it's used in some of the BF jobs. If the GIDs and permissions in devspace and the host happen to be compatible this PR doesn't do anything, otherwise it should ensure the docker nodes work regardless of your user-id

[joshmoore]

I can see the benefit moving forward for new devspaces, but I'm concerned about how to roll out this (and perhaps some of the other changes) to a running devspace. If it's not required, and they are currently working, perhaps we hold this for 0.12 (assuming your current batch of PRs is targeting 0.11)

[manics]

Sounds fine to me