Update elixir to link to alternative and similarly official images

[adamu]

The official Elixir docker image is maintained by the Erlang Ecosystem Foundation, but there are alternative images maintained by Hex, Elixir's official package manager.

There are a number of issues with the official image at the moment:

• Updated manually, so there is a few days delay between a new release, and image availability. E.g. see this elixir forum post.
• Issues with breaking changes for existing tags. Here are some examples of issues reported:
  • 1.13-alpine rebuilt 10 hours ago now getting errors erlef/docker-elixir#41
  • Latest elixir:1.13.4 breaks SSL options for Postgrex. erlef/docker-elixir#50
  • docker build error for 1.10.4-alpine erlef/docker-elixir#23

The images produced by Hex do not have these issues because they are built automatically whenever there is a new version and the tags are immutable - if a user wants to update a dependency, they need to update the relevant tag.

Because of this image's offical status, many users are unaware of the alternative option, which for Elixir deployments can often be the preferred choice, and is the one recommended by the Phoenix framework. This PR breifly mentions the issue about breaking changes, and links to the hexpm images so people have the information to choose the solution that works best for them.

I also updated the get-help section to link to the Elixir repository wiki, which has a maintained list of help resources.

[josevalim]

Good call 👍