Docker entrypoint shell script that updates the docker user's uid to that of a specific directory, allowing that directory to be writable.

entrypoint-update-uid.sh

#!/bin/bash
# Docker entrypoint file to set the user's uid to an id that can write to
# a specific directory. Note: entrypoint must be run as root
#
# Usage:
#  Set MAIN_USERNAME to the username that you want the container to run as
#  Set MAIN_DIRECTORY to the directory that you want the user to be able to write

MAIN_USERNAME=username
MAIN_DIRECTORY=/home/username/project

# Any other entrypoint can be included or added at the bottom
# ==================================================
TARGET_GID=$(stat -c "%g" $MAIN_DIRECTORY)
TARGET_UID=$(stat -c "%u" $MAIN_DIRECTORY)
cat /etc/group | grep $TARGET_GID > /dev/null 2>&1
# Create new group using target GID and add nobody user
if [ $? -ne 0 ]; then
  groupadd -g $TARGET_GID tempgroup
  usermod -u $TARGET_UID -a -G tempgroup $MAIN_USERNAME
else
  # GID exists, find group name and add
  GROUP=$(getent group $TARGET_GID | cut -d: -f1)
  usermod -u $TARGET_UID -a -G $GROUP $MAIN_USERNAME
fi

sudo -u $MAIN_USERNAME exec "$@"