Summary:
This adds a two-stage Dockerfile for grist-core. The first stage builds
Grist, and the second collects all files needed to run Grist.
The resulting image is about 600 MB which is quite a bit bigger
than it needs to be, but seems fine for now when the first goal is
to establish that people can open and edit Grist files on their
own infrastructure.
The image uses stock python rather than our sandboxed python for now.
Test Plan: manual
Reviewers: dsagal
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D2637
