automate setting dockerSizeGb in images.json

[williamstein]

• I'm just manually setting this by looking at pages like https://hub.docker.com/repository/docker/sagemathinc/cocalc-docker/tags?page=1&ordering=last_updated

• Technically the value depends on the particular tag/version of a Docker image

• It would be good to have a way to automate updating this information periodically, especially if we use it in some important way (so far it is not used much).