drone create a user-defined network for the pipeline, and docker handles dns resolution differently for user-defined networks. You can test this out:
docker network create foo
docker run --network=foo -t -i alpine ping -c 1 openmediavault
my guess is that you will need to configure your host machine accordingly so that user-defined networks are able to properly resolve the dns.
furthermore, you would typically provide drone with the same public facing DNS that your end-users enter into their browser (e.g. github.openmediavault.com). In my experience, trying to use local dns is difficult to configure properly and prone to errors (like you are seeing).