Temp fix for database backup issue

Bumping the restic base image from 0.17.3 to 0.18.0 caused database backups to fail. See #44 for details.

I'm still working on a root cause fix for the new base image. For now, the base image has been reverted back to 0.17.3 so the new features in stack-back 1.4.0 have a working release.

Full Changelog: v1.4.0...v1.4.1