It’s likely a pathing issue with Docker when installed with snap, you’re better off installing it with the official documentation from Docker.
Remove docker from snap
snap remove docker
Remove the docker directory, and old version (It’s okay if these don’t exist already)
rm -R /var/lib/docker
sudo apt-get remove docker docker-engine docker.io
Install the official docker package: https://docs.docker.com/install/linux/docker-ce/ubuntu/
Update: Since posting this answer, I’ve learnt that tools installed using snap are installed in a sandbox with limited permissions outside of that sandbox. This is likely the cause as docker won’t have access to the external filesystem from its isolated sandbox environment.