This is for elixir latest stable image and next -dev image.
➸ docker images
REPOSITORY TAG IMAGE ID CREATED SIZE
elixir 1.2 fcc8b4432e3e About a minute ago 290.2 MB
elixir 1.3 5c67fece33fb 7 minutes ago 290.3 MB➸ docker run -it --rm elixir:1.3
Erlang/OTP 18 [erts-7.2.1] [source] [64-bit] [smp:8:8] [async-threads:10] [hipe] [kernel-poll:false]
Interactive Elixir (1.3.0-dev) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> System.version
"1.3.0-dev"
iex(2)> System.get_env "ELIXIR_VERSION"
"1.3.0-dev@dcef08d"
iex(3)> :os.getenv "ELIXIR_VERSION"
'1.3.0-dev@dcef08d'
iex(4)>