Make `BUNDLE_LOCATION` configurable via ARG

[jgsuess]

The bundle location is fixed in this image. As a consequence, caching outside the local directory is not possible without copying the cache in Gitlab CI. This disables this image for CI use in Gitlab.

Could you please add a Docker ARG to the Dockerfile that allows to set the env on image build, so that it can be directed to the default login directory.