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Tips & Tricks

Tips & Tricks

This page has tips & tricks for things I’ve run into with Docker.

Allow non-root user to use Docker

During installation, the user is given an option to add the current user’s account to the docker group to allow them to run docker/docker compose commands without using sudo.

If you get a permission error when trying to run docker commands, add your user account to the docker group:

sudo usermod -aG docker $USER

Helper Containers

“Helper” containers use a small base image like alpine or busybox to perform actions like backing up a database or moving files to/from the host/a container. Using this method, you can move files into a container without bringing down another container using it, or copy files out of the container.

Copy data to/from the host and a volume

Use the busybox container to facilitate copying files into a volume from the host, or out of a volume to the host.

docker run -v <volume_name>:/path/in/container --name helper busybox true  # Create the helper container
docker cp helper:/path/in/container ./path/on/host  # Copy data from path in container to the host
docker rm helper  # remove the container after operation
docker run -v <volume_name>:/path/in/container --name helper busybox true  # Create the helper container
docker cp ./path/on/host helper:/path/in/container  # Copy data from host to path in the helper container
docker rm helper  # remove the container after operation

Mount /var/lib/docker on another drive

Docker stores its data (images, caches, named volumes, etc) at /var/lib/docker. Over time, this path can become very large. If you have another drive available, you can safely move the /var/lib/docker mount to the new drive.

Example values

The guides below use the following values as examples, you should replace them with your own when you run the commands:

  • /dev/sdd: The drive to use as a mount for Docker data
  • /mnt/ex_path: The example path where we will mount the Docker data partition
  • /dev/sdd1: The drive’s partition where data will be stored

Fresh setup

If you do not currently have any data in /var/lib/docker, you can simply create an entry in your /etc/fstab and reload the systemd daemon. First, run lsblk -f to list all of your drives. Find the one you want to mount your Docker data in, i.e. /dev/sdd1, and copy its UUID.

Edit your /etc/fstab and use the UUID to change the /var/lib/docker mount:

UUID="..." /var/lib/docker ext4 defaults,nofail 0 0

You can use any path you want. /mnt is the conventional path for temporary filesystems, like external drives, or manual mounts like internal storage devices.

Run sudo systemctl daemon-reload && sudo mount -a or reboot your machine to start using the new mount.

Move existing data

If you already have existing Docker data in /var/lib/docker, you can move the data to another path and re-mount on a new drive.

First, back up the existing Docker directory:

sudo mv /var/lib/docker /var/lib/docker.bak

Find the external drive you want to use for Docker’s data. Note the drive’s path and UUID, i.e. /dev/sdd1:

lsblk -f

Create a temporary mount point to move the data to; you will move it back to /var/lib/docker after remounting the path on the new drive:

sudo mkdir -p /mnt/ex_path

Mount the new drive to the temporary /mnt/ex_path path:

sudo mount /dev/sdd1 /mnt/ex_path

Before moving any data, you should make sure Docker is completely stopped to avoid copying data while the container is still in use and risking corruption. Stop Docker with:

sudo systemctl stop docker docker.socket

Copy existing Docker data to the temporary path we created:

sudo rsync -aHAXx /var/lib/docker/ /mnt/ex_path/

Move the existing Docker data path to a backup path:

sudo mv /var/lib/docker /var/lib/docker.bak

Then recreate the /var/lib/docker path:

sudo mkdir -p /var/lib/docker

Mount the new drive at Docker’s data path. Docker will begin using this path for data once it’s restarted:

sudo mount /dev/sdd1 /var/lib/docker

Persist the mount by adding it to your /etc/fstab. This ensures the drive remounts on a system reboot. Get the drive UUID by running sudo blkid and find the drive you want to use (i.e. /dev/sdd1), i.e. /dev/sdd1: UUID="abc-123" TYPE="ext4". Edit your /etc/fstab file and add a new entry:

UUID=abc-123 /var/lib/docker ext4 defaults,nofail 0 2

Explanation of the mount options used

  • UUID=abc-123: Identifies the disk partition to mount. Using a UUID is preferred over /dev/sdd.
  • /var/lib/docker: The mount point where Docker data will be stored.
  • ext4: Filesystem type on the partition. Could also be xfs, btrfs, etc.
  • Mount options (defaults,nofail):
    • defaults: Enables default mount behavior. Equivalent to:
      • rw,suid,dev,exec,auto,nouser,async
    • nofail: Prevents boot failure if the drive is missing.
    • 0: Dump flag (legacy backup utility). Usually left as 0.
    • 2: Filesystem check order during boot (fsck).

Reload the systemctl daemon & mount all drives in /etc/fstab:

sudo systemctl daemon-reload
sudo mount -a

Restart Docker with sudo systemctl start docker, then verify Docker data exists at the new path:

docker ps -a
docker images

If everything looks good, you can remove the backup you created:

sudo rm -rf /var/lib/docker.bak
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