fix(docker): bump elixir image version

[vitor-mda]

Hey there!

I bumped into an issue when building the current Dockerfile. A 404 error happens when fetching the stretch release from the Debian repo:

This is happening because the base Docker image depends on the stretch release living on the regular Debian repo, which has now been moved to the archived one. Bumping the image version fix this issue.

Also, I'd just like to say I really loved the work done on introducing Elixir to newcomers in this repo. Awesome stuff! ❤️