Makes docker images default to non-root execution (#1031)

De-escalates to a normal user when the docker image is run as root.

Allows GRIST_DOCKER_USER and GRIST_DOCKER_GROUP to be passed to override the default de-escalation behaviour.

Backwards compatible with previous root installations.

--------

This change adds a new docker_entrypoint.sh, which when run as root de-escalates to the provided user, defaulting to grist:grist. This is similar to the approach used by the official postgres docker image.

To achieve backwards compatibility, it changes ownership of any files in `/persist` to the user it's given at runtime. Since the docker container is typically run as root, this should always work.

If the container is run as a standard user from the very start:
* It's the admin's responsibility to ensure `/persist` is writable by that user.
* `/grist` remains owned by root and is read-only.

Dockerfile

@@ -122,6 +122,15 @@ RUN \
   mv /grist/static-built/* /grist/static && \
   rmdir /grist/static-built
 
+# To ensure non-root users can run grist, 'other' users need read access (and execute on directories)
+# This should be the case by default when copying files in.
+# Only uncomment this if running into permissions issues, as it takes a long time to execute on some systems.
+# RUN chmod -R o+rX /grist
+
+# Add a user to allow de-escalating from root on startup
+RUN useradd -ms /bin/bash grist
+ENV GRIST_DOCKER_USER=grist \
+    GRIST_DOCKER_GROUP=grist
 WORKDIR /grist
 
 # Set some default environment variables to give a setup that works out of the box when
@@ -151,5 +160,5 @@ ENV \
 
 EXPOSE 8484
 
-ENTRYPOINT ["/usr/bin/tini", "-s", "--"]
+ENTRYPOINT ["./sandbox/docker_entrypoint.sh"]
 CMD ["node", "./sandbox/supervisor.mjs"]