This is the process of setting up erlang, rebar3, and cowboy for a Hello World, starting with a clean Debian 8 install.
Update apt and install deps:
root@046edcaea45a:~# apt-get update
root@046edcaea45a:~# apt-get install erlang erlang-dev gcc
root@046edcaea45a:~# wget https://s3.amazonaws.com/rebar3/rebar3
root@046edcaea45a:~# mkdir ~/bin/
root@046edcaea45a:~# mv rebar3 ~/bin/
root@046edcaea45a:~# chmod +x ~/bin/rebar3