Free Disk Space Used by Docker

TL;DR use any/all of docker system prune, docker container prune, docker image prune, docker volume prune or docker network prune to free the according space.

Use docker system df to look at the disk space used by docker, to see if it is worth it, or to be better informed. I got this output:

> docker system df
TYPE                TOTAL               ACTIVE              SIZE                RECLAIMABLE
Images              125                 12                  15.27GB             15.03GB (98%)
Containers          53                  0                   12.6GB              12.6GB (100%)
Local Volumes       24                  9                   344.2MB             295.4MB (85%)
Build Cache         169                 0                   5.8GB               5.8GB

Now that you know, you can free the memory with:

> docker system prune
WARNING! This will remove:
  - all stopped containers
  - all networks not used by at least one container
  - all dangling images
  - all dangling build cache

Are you sure you want to continue? [y/N] y
Deleted Containers:
0b6aafd9ce1d346bec7470f6042b6e1b30516973e096ef1954ccd823f9bb846a
...

After that no containers to be removed anymore, proof:

> docker container prune
WARNING! This will remove all stopped containers.
Are you sure you want to continue? [y/N] y
Total reclaimed space: 0B

Run docker image prune --all to gain a lot of disk space with the trade off of removing all your images, which will be downloaded when needed again.