Since October 2017 the default config name no longer has an underscore
but a dash in the filename. See osmo-sgsn.git Change-Id
If804da17a7481e79e000fe40ae0d9c4be9722e61
Let's use the "ADD" trick that we use for git repositories also for
the osmocom:nightly debian package feed. This way any change to the
package feed will result in a rebuild of the container.