How do I know which images I can delete?
$ docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
discourse/base 2.0.20180802 d6f8b6029227 4 weeks ago 1.74GB
local_discourse/web_only latest 0301af96b2e8 5 months ago 2.95GB
local_discourse/data latest a8b8c3da644a 8 months ago 1.85GB
local_discourse/mail-receiver latest a322c9207234 8 months ago 142MB
discourse/base 2.0.20171204 64d62a045a4e 8 months ago 1.81GB
discourse/mail-receiver 1.1.2 44042627246b 14 months ago 142MB
samsaffron/docker-gc latest 54ca424ca8d6 2 years ago 57.7MB