Docker - Permission Denied - user 1000? - Setting PUID/PGID?

I am trying to deploy the Ghost CMS docker image and using a similar tutorial like one I used to start a Next Cloud docker image. I want the Ghost CMS running on its own bridge network.

I believe I understand everything and I have a dedicated user created for Ghost CMS. Also have mariadb running on the same bridge. However, after setting PUID/PGID the ghost image shuts down immediately with a permission error: "mkdir: cannot create directory ‘var/lib/ghost’: Permission denied. When I go to check the owner of the directory, it reads “1000” after this failure. Is this image unable to use a different PUID/PGID? How can I get around this? Do I need to create a 1000 user ID?

Can anyone link me to a tutorial that will help out? Trying to do all of this from Synology’s GUI.