What is the right way to automount an external drive in a docker container?

I want to create a personal cloud service in a docker container on a Raspberry Pi. Due to the limited storage and longevity of SD cards, I want to create the database one an external drive of sufficient capacity.

First I ran into the issue of mounting at all in container and could solve this problem by creating a privileged container. Now I wanted to use autofs.service to automatically mount the drive to desired location, but systemd is missing. After some reading I found that using it in a container is not recommended.

So how do I automount a drive the easiest way? Which is the way to go?

I believe the correct way to do this is by mounting the external drive on your host system, and then mapping the volume onto the container at startup.

I'll provide a basic example with the NextCloud container.

So mount your external drive normally on your host system - let's say you mount it to /mnt/my-data.

Next, you want to create a directory for the NextCloud data:

$ mkdir /mnt/my-data/nc-data

Then, when running your NextCloud container image, provide information to map the volume onto the internal NextCloud data directory:

$ docker run -d \
-v /mnt/my-data/nc-data:/var/www/html \
nextcloud

If you want an external database, do the same for MariaDB:

$ mkdir /mnt/my-data/nc-mariadb

$ docker run -d \
-v /mnt/my-data/nc-mariadb:/var/lib/mysql \
mariadb

Make sure the owner and permissions for the data directories match those of the container running.

I find that it's easiest to use docker-compose to generate a complete installation file of the entire container stack. Using the above example, and the docker-compose file shown on Docker Hub, this would be the result:

version: '2'

services:
  db:
    image: mariadb
    restart: always
    command: --transaction-isolation=READ-COMMITTED --binlog-format=ROW
    volumes:
      - /mnt/my-data/nc-mariadb:/var/lib/mysql
    environment:
      - MYSQL_ROOT_PASSWORD=
      - MYSQL_PASSWORD=
      - MYSQL_DATABASE=nextcloud
      - MYSQL_USER=nextcloud

  app:
    image: nextcloud
    restart: always
    ports:
      - 8080:80
    links:
      - db
    volumes:
      - /mnt/my-data/nc-data:/var/www/html
    environment:
      - MYSQL_PASSWORD=
      - MYSQL_DATABASE=nextcloud
      - MYSQL_USER=nextcloud
      - MYSQL_HOST=db

Note that the "volumes" section is not present, since we map an absolute path on your host system (/mnt/my-data/nc-xxxx) and not a named volume.