(core) fix some rusting of the grist-core build

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

.dockerignore

@@ -10,4 +10,5 @@
 !static
 !bower_components
 !sandbox
+!plugins
 !test