feat: properly export env vars for cnb apps

They are not baked into the image like they are for herokuish, so we need to manually set them via --env. Additionally, due to newline constraints in docker, we need to use `--env=key` format to correctly pull the values into place.

tests/unit/test_helper.bash

@@ -513,6 +513,13 @@ add_release_command() {
   echo "release: touch /app/release.test" >> "$APP_REPO_DIR/Procfile"
 }
 
+add_requirements_txt() {
+  local APP="$1"
+  local APP_REPO_DIR="$2"
+  [[ -z "$APP" ]] && local APP="$TEST_APP"
+  echo "flask" >> "$APP_REPO_DIR/requirements.txt"
+}
+
 build_nginx_config() {
   # simulate nginx post-deploy
   dokku domains:setup "$TEST_APP"