Cannot mount Config directory in Nextcloud Docker container - docker

Question

I'm trying to create a custom Nextcloud config locally, then have the ability to mount it to the appropriate folder using volumes as defined here: https://github.com/nextcloud/docker#persistent-data. All the volume mounts work except for the config mount... Why is that being treated differently here?
Steps to reproduce
0) Enter a new/emptry directory (containing no sub-directories or additional files).
1) Create a docker-compose.yml file containing only the below contents:
version: "3.4"
services:
nextcloud:
image: nextcloud:latest
volumes:
- "./nextcloud/custom_apps:/var/www/html/custom_apps"
- "./nextcloud/config:/var/www/html/config"
- "/data/nextcloud:/var/www/html/data"
- "./themes:/var/www/html/themes"
2) docker-compose up -d
Expected behavior
Work. I should be able to see the /var/www/html/config contents locally at ./nextcloud/config, and then insert a customer config.php, which is then updated within the container.
Actual behavior
An ERROR when bringing up the container, specific to the config directory. If I remove the ./nextcloud/config:/var/www/html/config volume mount above, then the container will start without error.
ERROR message
ERROR: for nextcloud Cannot start service nextcloud: OCI runtime create failed: container_linux.go:349: starting container process caused "process_linux.go:449: container init caused \"rootfs_linux.go:58: mounting \\"/home/user/Nextcloud-test/nextcloud/config\\" to rootfs \\"/var/lib/docker/overlay2/41b567141e23b16cf5e4f99f4c33703fc9a533aa5a4bef68fbba70a74842ca88/merged\\" at \\"/var/lib/docker/overlay2/41b567141e23b16cf5e4f99f4c33703fc9a533aa5a4bef68fbba70a74842ca88/merged/var/www/html/config\\" caused \\"not a directory\\"\"": unknown: Are you trying to mount a directory onto a file (or vice-versa)? Check if the specified host path exists and is the expected type
ERROR: Encountered errors while bringing up the project.
Server configuration
Operating system:
Operating System: Debian GNU/Linux 10 (buster)
Kernel: Linux 4.19.0-8-cloud-amd64
Architecture: x86-64
Image
nextcloud:latest (apache)

Answer 1

I could not reproduce using your steps (Ubuntu 18.04).
From here, running docker-compose up -d then docker-compose logs, I see no errors. Then, when running docker inspect on the container, I see the following:
...
"Volumes": {
"/var/www/html": {},
"/var/www/html/config": {},
"/var/www/html/custom_apps": {},
"/var/www/html/data": {},
"/var/www/html/themes": {}
},
...
Which suggests the mount has worked without problem.
What I suggest you do:
Check the directory ./nextcloud/config exists and is not a file
Check your Docker and Docker Compose installation is up-to-date
Try running the Docker container with docker run -it -v ./nextcloud/config:/var/www/html/config <containername> /bin/bash to explore if the mount works manually
Try to do the same on a minimal example such as the Getting started example