Makes docker images support non-root execution

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"]

sandbox/docker_entrypoint.sh

@@ -0,0 +1,40 @@
+#!/usr/bin/env bash
+set -Eeuo pipefail
+
+# Runs the command provided as arguments, but attempts to configure permissions first.
+
+important_read_dirs=("/grist" "/persist")
+write_dir="/persist"
+current_user_id=$(id -u)
+
+# We want to avoid running Grist as root if possible.
+# Try to setup permissions and de-elevate to a normal user.
+if [[ $current_user_id == 0 ]]; then
+  target_user=${GRIST_DOCKER_USER:-grist}
+  target_group=${GRIST_DOCKER_GROUP:-grist}
+
+  # Make sure the target user owns everything that Grist needs write access to.
+  find $write_dir ! -user "$target_user" -exec chown "$target_user" "{}" +
+
+  # Restart as the target user, replacing the current process (replacement is needed for security).
+  # Alternative tools to setpriv are: chroot, gosu.
+  # Need to use `exec` to close the parent shell, to avoid vulnerabilities: https://github.com/tianon/gosu/issues/37
+  exec setpriv --reuid "$target_user" --regid "$target_group" --init-groups /usr/bin/env bash "$0" "$@"
+fi
+
+# Validate that this user has access to the top level of each important directory.
+# There might be a benefit to testing individual files, but this is simpler as the dir may start empty.
+for dir in "${important_read_dirs[@]}"; do
+  if ! { test -r "$dir" ;} ; then
+    echo "Invalid permissions, cannot read '$dir'. Aborting." >&2
+    exit 1
+  fi
+done
+for dir in "${important_write_dirs[@]}"; do
+  if ! { test -r "$dir" && test -w "$dir" ;} ; then
+    echo "Invalid permissions, cannot write '$dir'. Aborting." >&2
+    exit 1
+  fi
+done
+
+exec /usr/bin/tini -s -- "$@"