(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

Dockerfile

@@ -30,23 +30,25 @@ RUN \
 ################################################################################
 
 # Now, start preparing final image.
-FROM node:14-slim
+FROM node:14-buster-slim
 
 # Copy node files.
 COPY --from=builder /node_modules node_modules
 COPY --from=builder /_build _build
 COPY --from=builder /static static
 
-# Copy python files.
+# Copy python files. TODO: package python3.9 also in grist-core.
 COPY --from=builder /usr/bin/python2.7 /usr/bin/python2.7
 COPY --from=builder /usr/lib/python2.7 /usr/lib/python2.7
 COPY --from=builder /usr/local/lib/python2.7 /usr/local/lib/python2.7
+RUN ln -s /usr/bin/python2.7 /usr/bin/python
 
 # Add files needed for running server.
 ADD package.json package.json
 ADD ormconfig.js ormconfig.js
 ADD bower_components bower_components
 ADD sandbox sandbox
+ADD plugins plugins
 
 # Keep all storage user may want to persist in a distinct directory
 RUN mkdir -p /persist/docs
@@ -57,8 +59,8 @@ RUN mkdir -p /persist/docs
 # Variables will need to be overridden for other setups.
 ENV GRIST_ORG_IN_PATH=true
 ENV GRIST_HOST=0.0.0.0
-ENV APP_HOME_URL=http://localhost:8484
-ENV APP_DOC_URL=http://localhost:8484
+ENV GRIST_SINGLE_PORT=true
+ENV GRIST_SERVE_SAME_ORIGIN=true
 ENV GRIST_DATA_DIR=/persist/docs
 ENV GRIST_SESSION_COOKIE=grist_core
 ENV TYPEORM_DATABASE=/persist/home.sqlite3