Switch to hexpm/elixir and Fuly Qualified Tag Names

[restlessronin]

I really like being able to specify the version of every piece of the runtime (elixir, erlang and linux). The official elixir docker image does not tag images with this much information.

However, the hexpm/elixir images do provide this information in the tags. May I suggest that we switch to those images, and provide more detailed tag names on the nicbet images? We should actually even make the tag names longer and include the Node version (which is already parameterized along with the phoenix version).

If this makes sense, I can work on a PR for the dockerfile, but I don't believe I'm able to do the CI work.