I found this in the documentation:

rel/env.sh.eex and rel/env.bat.eex - template files that are copied into every release and are executed on every command to set up environment variables, including specific ones to the VM, and the general environment

So I believe so.

A Phoenix release with runtime.exs will also have access to all environment variables like:

PORT=4000 build/prod/rel/new_phoenix/bin/new_phoenix start

I talked about it in my latest post:

If you want to export all variables from an environment file that doesn’t come with the release, you can do the following in Bash:

set -o allexport
source env.sh
set +o allexport
build/prod/rel/new_phoenix/bin/new_phoenix start