Branch stable includes the latest stable version of Elixir Erlang. This branch will be tagged as latest in docker.
Branch master will build Elixir and Erlang from the latest master on their git repos. This branch will be tagged as edge in docker.
The edge image will automatically built with every new commit to elixir/elixir-lang and erlang/otp. See blog post for behind the scenes.
The Docker container provides a info.txt in the root directory with details about the build