Add -buster tags to each image variant

[nickjj]

A lot of other programming runtimes like Python, Ruby and Node explicitly have -buster tags in the list of tags that appear on the Docker Hub.

I think Elixir should too.

Currently you have image tags like elixir:1.9-slim but it's beneficial to have elixir:1.9-slim-buster because now at a glance we can see the underlying image is using Buster.

At the time of making this comment, it is using Buster under the hood because Buster is the latest stable release of Debian but for a while it was Stretch and now as an end user we don't know which one is being used unless we look at the Dockerfile and trace through the base image chain until eventually arriving at the Erlang image.

[jayjun]

When I bump

FROM elixir:1.9.2-slim AS build
...
FROM erlang:22-slim AS app

to elixir:1.10.1-slim, my containers fail with

/app/erts-10.6.4/bin/beam.smp: error while loading shared libraries: libtinfo.so.6: cannot open shared object file: No such file or directory

Both images are based on erlang:22-slim. However, my app was built on erlang:22.2-slim (Buster) then ran on erlang:22.1-slim (Stretch) cached earlier.

-buster-slim avoids this but the bigger problem is mutable tags.

Every Elixir image is built off an exact Erlang version anyway. Why not make it explicit with FROM erlang:22.2.8-slim? That way we know which Erlang image to run releases on.

Related https://github.com/erlang/docker-erlang-otp/commit/966a8b993e8479950aa7cfc09570d169de2b6bff.