BUNDLE_APP_CONFIG overrides default location for .bundle/config

Setting BUNDLE_APP_CONFIG in the Dockerfile is causing issues when a repo is copied or mounted into the image. The default location is normally the root of the app repo, e.g. /app/.bundle/config. However, since BUNDLE_APP_CONFIG is set, it overrides this default behavior and looks for bundle config in BUNDLE_APP_CONFIG which is set to /usr/local/bundle/config.

This causes problems because:

• Our hardened image changes to a more restricted user, appuser, which wouldn't have access to /usr/local/bundle/config causing permission issues.
• It is not possible to unset environment variables in Dockerfiles so it cannot be fixed on our end. To workaround this we have unset BUNDLE_APP_CONFIG throughout different jobs in our pipeline.

Can we please remove line 40 in the Dockerfile?