Fix docker graceful shutdown (#830)

* run.sh: Replace pid with nodejs's one using exec

This notably ensures that "docker stop" sends a TERM signal to the node
process which can handle it gracefully and avoid document corruption.

* Use exec form for CMD in Dockerfile

---------

Co-authored-by: Florent FAYOLLE <florent.fayolle@beta.gouv.fr>

Dockerfile

@@ -142,4 +142,4 @@ ENV \
 
 EXPOSE 8484
 
-CMD ./sandbox/run.sh
+CMD ["./sandbox/run.sh"]

sandbox/run.sh

@@ -16,4 +16,4 @@ if [[ "$GRIST_SANDBOX_FLAVOR" = "gvisor" ]]; then
   ./sandbox/gvisor/update_engine_checkpoint.sh
 fi
 
-NODE_PATH=_build:_build/stubs:_build/ext node _build/stubs/app/server/server.js
+exec env NODE_PATH=_build:_build/stubs:_build/ext node _build/stubs/app/server/server.js