The parameter -e sets environment variables. googling for environment variable PUID yields among others this result:
Understanding PUID and PGID on linuxserver.io, who provide a docker image for calibre-web.
Quote from that page:
Using the PUID and PGID allows our containers to map the container's internal user to a user on the host machine. All of our containers use this method of user mapping and should be applied accordingly.
Another note from that page:
We are aware that recent versions of the Docker engine have introduced the --user flag. Our images are not yet compatible with this, so we recommend continuing usage of PUID and PGID.
So, it's needed to run the container. How this is implemented is up to the maintainers of those images.
