Summary:
* Base docker image no longer contained a `python` binary.
Made a small fix for this, with proper python3 packaging
in the works separately.
* Added missing plugins directory for importing csv+xlsx.
* Tweaked environment variables to avoid needing to hard-code
addresses, which was burdensome for single-server hosts.
Test Plan:
Tested manually. It would be good to move over some
fraction of our tests to catch packaging glitches, or to run
our standard deployment tests on a deployment derived from
grist-core.
Reviewers: jarek
Reviewed By: jarek
Subscribers: jarek
Differential Revision: https://phab.getgrist.com/D3159