vscode/Dockerfile: Add sudo, leave /tmp alone

[SamB]

The official gitpod Docker images drop root now, so add sudo throughout.

Also, don't try to clear out /tmp: we're using tmpfs now, and you never know when something is actually using /tmp while the system is still running, so it's both safer and faster to just leave it.

Tested at: https://github.com/SamB/vscode/commit/65df82546e7869e5c9c2dbc545d8e4b9e9862939