By default, Docker storage its files in the /var/lib/docker directory in the file system. If, like me, you're running out of disk space in the default file system, you can quickly change the default directory. This are the steps I followed in Debian 10, it should work with any distro that uses systemd

As superuser, modify the systemd docker startup script. Edit the file /lib/systemd/system/docker.service replacing the ExecStart command:

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$ sudo vim /var/systemd/system/docker.service

Change the line:

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ExecStart=/usr/bin/docker daemon -H fd://

to

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ExecStart=/usr/bin/docker daemon -g /new/path -H fd://

then stop the service, and be sure that the daemon is completely stopped:

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$ sudo systemctl stop docker
$ ps -aux | grep -i docker

The only output should be the one from grep. Then you can reload the system and -optionally- synchronize your current docker data to the new directory

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$ sudo systemctl daemon-reload
$ sudo mkdir /new/path
$ sudo rsync -aqxP /var/lib/docker /new/path

Your docker installation should be running in the new data directory:

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$ ps -aux | grep -i docker

That's it.