(core) freshen core README; support python3 in grist-core docker image

Summary:
This updates the grist-core README to list specific features of Grist,
to make it easier for a casual visitor to get a sense of its scope. Adds links
to some new resources (reviews, templates, grist v airtable post) that could
also help. Adds python3 to docker image so that templates work without fuss.

Test Plan: existing tests should pass

Reviewers: georgegevoian

Reviewed By: georgegevoian

Subscribers: dsagal, anaisconce

Differential Revision: https://phab.getgrist.com/D3204

Dockerfile

@@ -1,5 +1,5 @@
 ################################################################################
-## Build stage
+## Javascript build stage
 ################################################################################
 
 FROM node:14-buster as builder
@@ -18,12 +18,21 @@ ADD static static
 ADD test/tsconfig.json test/tsconfig.json
 RUN yarn run build:prod
 
+################################################################################
+## Python collection stage
+################################################################################
+
+# Fetch python3.9 and python2.7
+FROM python:3.9-slim-buster as collector
+
 # Install all python dependencies.
 ADD sandbox/requirements.txt requirements.txt
+ADD sandbox/requirements3.txt requirements3.txt
 RUN \
   apt update && \
-  apt install -y python-pip && \
-  pip install -r requirements.txt
+  apt install -y --no-install-recommends python2 python-pip python-setuptools && \
+  pip2 install -r requirements.txt && \
+  pip3 install -r requirements3.txt
 
 ################################################################################
 ## Run-time stage
@@ -32,16 +41,29 @@ RUN \
 # Now, start preparing final image.
 FROM node:14-buster-slim
 
+# Install libexpat1, libsqlite3-0 for python3 library binary dependencies.
+RUN \
+  apt-get update && \
+  apt-get install -y --no-install-recommends libexpat1 libsqlite3-0 && \
+  rm -rf /var/lib/apt/lists/*
+
+# Keep all storage user may want to persist in a distinct directory
+RUN mkdir -p /persist/docs
+
 # Copy node files.
 COPY --from=builder /node_modules node_modules
 COPY --from=builder /_build _build
 COPY --from=builder /static static
 
-# 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
+# Copy python files.
+COPY --from=collector /usr/bin/python2.7 /usr/bin/python2.7
+COPY --from=collector /usr/lib/python2.7 /usr/lib/python2.7
+COPY --from=collector /usr/local/lib/python2.7 /usr/local/lib/python2.7
+COPY --from=collector /usr/local/bin/python3.9 /usr/bin/python3.9
+COPY --from=collector /usr/local/lib/python3.9 /usr/local/lib/python3.9
+COPY --from=collector /usr/local/lib/libpython3.9.* /usr/local/lib/
+# Set default to python3
+RUN ln -s /usr/bin/python3.9 /usr/bin/python && ldconfig
 
 # Add files needed for running server.
 ADD package.json package.json
@@ -50,13 +72,11 @@ 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
-
 # Set some default environment variables to give a setup that works out of the box when
 # started as:
 #   docker run -p 8484:8484 -it <image>
 # Variables will need to be overridden for other setups.
+ENV PYTHON_VERSION_ON_CREATION=3
 ENV GRIST_ORG_IN_PATH=true
 ENV GRIST_HOST=0.0.0.0
 ENV GRIST_SINGLE_PORT=true