Update readme on docker hub

[straight-shoota]

The readme at https://hub.docker.com/r/crystallang/crystal is outdated.

crystal-lang/crystal-dist#14 suggested an automation to update the readme from the Crystal repository. We could to this and integrate it as a step in docker/apply-latest-tags.sh.

However, I think it would be better to provide a readme specific to the docker repository, which explains the available image types. We might still want to have that automated to always show the latest tags, though.

[bcardiff]

I would rather focus on publishing a Docker official image and mimic whatever the other languages are doing. See https://github.com/docker-library.

Eventually we could leave crystallang/crystal for CI and maybe nightlies.

[straight-shoota]

Yes, that's already tracked in #126.

That doesn't mean we shouldn't have to update the readme, though.

[stellarpower]

Please could you add a comment as to the difference between the -dev and normal tags as part of this? Without diving into the filesystem or build scripts, it's not clear to me what the difference between the two is. FWIW, a number of projects copy across their readmes from source hosting to their docker hub pages, but personally I find it much more useful when the docker (or other image) pages describe specifically what happens as part of the image as opposed to a general overview of the project, so I think whatever happens re the above, adding more on this would be really helpful.

Cheers

[straight-shoota]

The Docker hub has been manually updated to include information about tag naming schemes.

Automatically updated information about available tags (via crystal-lang/crystal-dist#14) would be a nice enhancement.