GIT_DISCOVERY_ACROSS_FILESYSTEM not set

src : https://git.pleroma.social/pleroma/pleroma

remote:        * Stack heroku-18
remote:        * Erlang 21.2
remote:        * Elixir 1.7

$ cat elixir_buildpack.config
erlang_version=21.2
elixir_version=1.7
always_rebuild=false
runtime_path=/app

$ git push heroku master
remote: -----> Fetching app dependencies with mix
remote: fatal: not a git repository (or any parent up to mount point /)
remote: Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).

$GIT_DIR is empty, $git_dir_value is also empty.

There is no .git directory in $build_path.

lib/app_funcs.sh

# https://github.com/HashNuke/heroku-buildpack-elixir/blob/master/lib/app_funcs.sh#L71
function app_dependencies() {
  # Unset this var so that if the parent dir is a git repo, it isn't detected
  # And all git operations are performed on the respective repos
  local git_dir_value=$GIT_DIR
  unset GIT_DIR

  cd $build_path
  output_section "Fetching app dependencies with mix"
  mix deps.get --only $MIX_ENV || exit 1

  export GIT_DIR=$git_dir_value
  cd - > /dev/null
}

Start with lib/app_funcs.sh and the app_dependencies function, then reproduce the Heroku build using the shown elixir_buildpack.config and mix deps.get flow. Check how GIT_DIR is unset and restored around the dependency fetch. Done means dependencies fetch without the GIT_DISCOVERY_ACROSS_FILESYSTEM error while the existing Git environment behavior remains intact.