include pyodide in the docker image (#1019)

Grist has for some time supported a sandbox based on pyodide.
It is a bit slower to start than the gvisor-based sandbox, but
can run in situations where it can't. Until now it hasn't been
easy to use when running Grist as a container, since the support
files weren't included. This change rectifies that omission.

Nothing changes by default. But now if you start Grist as a container
and set `GRIST_SANDBOX_FLAVOR=pyodide`, it should work rather than
fail.

Dockerfile

@@ -35,6 +35,12 @@ COPY stubs /grist/stubs
 COPY buildtools /grist/buildtools
 RUN yarn run build:prod
 
+# Prepare material for optional pyodide sandbox
+COPY sandbox/pyodide /grist/sandbox/pyodide
+COPY sandbox/requirements3.txt /grist/sandbox/requirements3.txt
+RUN \
+  cd /grist/sandbox/pyodide && make setup
+
 ################################################################################
 ## Python collection stage
 ################################################################################
@@ -108,6 +114,9 @@ ADD sandbox /grist/sandbox
 ADD plugins /grist/plugins
 ADD static /grist/static
 
+# Make optional pyodide sandbox available
+COPY --from=builder /grist/sandbox/pyodide /grist/sandbox/pyodide
+
 # Finalize static directory
 RUN \
   mv /grist/static-built/* /grist/static && \