Best not to use bind mounts with volumes. Better to use bindfs outside of docker to mount them elsewhere with an owner map. Install if need be

on host make a nextcloud user with id 82 (which is www-data in fpm-alpine container)

must run bindfs as sudo for mapping

sudo bindfs --map=nextcloud/sysadmin /var/lib/docker/volumes/nextcloud_user-files/_data /data/nextcloud/user-files sudo bindfs --map=nextcloud/sysadmin /var/lib/docker/volumes/nextcloud_src/_data /data/nextcloud/src

sudo mv sudo bindfs -o nonempty --map=sysadmin/nextcloud /data/nextcloud/user-files /var/lib/docker/volumes/nextcloud_user-files/_data

sudo bindfs -u nextcloud -g nextcloud /data/nextcloud/user-files /var/lib/docker/volumes/nextcloud_user-files/_data bindfs -u nextcloud -g nextcloud /data/nextcloud/src /var/lib/docker/volumes/nextcloud_user-src/_data

adding container directory

apk add bindfs --repository=http://dl-cdn.alpinelinux.org/alpine/edge/testing addgroup --gid 1000 host &&
adduser -u 1000 -G host -g "" -D -H host &&
mkdir -p /source &&
mkdir -p /user-files &&
bindfs --force-user=host --force-group=host --create-for-user=www-data --create-for-group=www-data --chown-ignore --chgrp-ignore /var/www/html /source &&
bindfs --force-user=host --force-group=host --create-for-user=www-data --create-for-group=www-data --chown-ignore --chgrp-ignore /var/www/data /user-files