Prefer to use the image from the Hexpm team instead of the Docker team, the reason below was mentioned on the Elixir Forums:

• Image tags are not immutable.
• Delay in the availability of new versions.
• Cannot pick the combination of Elixir and Erlang versions you want.

Refer:

• hexpm/elixir
• hexpm/erlang

Example Dockerfile:

FROM hexpm/elixir:1.11.0-erlang-23.1.1-alpine-3.12.0 AS build

....
FROM alpine:3.12.0 AS app
...

COPY --from=build --chown=nobody:nobody /app/_build/prod/rel/app ./
...

Based on the image tag, it's clear which versions we use in the Build Release step

• Elixir: 1.11.0
• Erlang: 23.1.1
• Alpine: 3.12.0

Easily we can choose the OS to run the app on, that is alpine:3.12.0