Version variants without OS release version should point to latest

[fabianfrz]

for example ruby:2.4 points to jessie but in my opinion it should point to stretch and jessie should be available as ruby:2.4-jessie.

[FreekingDean]

Similarly for ruby:2.4-alpine should point to alpine:3.7 not alpine:3.4

[tianon]

This is along the same lines as docker-library/php#504 -- I think for the Alpine variants at least, we've probably had the new Alpine-version-specific variants for long enough that we can safely make this change without a transition period, but I'm not convinced the same applies to the bare ruby:X.Y tags (since they don't have an implied distribution already like the bare Alpine variants do).

[FreekingDean]

That makes sense. Sorry for adding a PR to the higher level repo before. I thought that adding code is easier to spark conversation :(

[FreekingDean]

Which is why #205 can be a talking point specifically around using the latest for alpine :)

[yosifkit]

This was fixed in docker-library/official-images#4385. See the ruby library file in the PR.