5dockerimagehistory<image># layers and their sizes
6dockerrmi<image:tag># remove an image (or just that tag)
7dockerrmi-f<image-id># force, even with multiple tags
8dockerimageprune-a# remove all unused images
Reading the output
REPOSITORY:TAG is the name; several tags can share one IMAGE ID
SIZE is the uncompressed size, and layers shared between images are counted in each, so the total overstates real disk use. Use docker system df for the true number
Gotchas
image is being used by stopped container: remove the container first (docker rm), or use -f
Removing a tag does not free space until the last tag pointing to that image is gone