Summary:
This adds support for gvisor sandboxing in core. When Grist is run outside of a container, regular gvisor can be used (if on linux), and will run in rootless mode. When Grist is run inside a container, docker's default policy is insufficient for running gvisor, so a fork of gvisor is used that has less defence-in-depth but can run without privileges.
Sandboxing is automatically turned on in the Grist core container. It is not turned on automatically when built from source, since it is operating-system dependent.
This diff may break a complex method of testing Grist with gvisor on macs that I may have been the only person using. If anyone complains I'll find time on a mac to fix it :)
This diff includes a small "easter egg" to force document loads, primarily intended for developer use.
Test Plan: existing tests pass; checked that core and saas docker builds function
Reviewers: alexmojaki
Reviewed By: alexmojaki
Subscribers: alexmojaki
Differential Revision: https://phab.getgrist.com/D3333
