enable CI builds of .deb files
We want the packaging repo to automatically build .deb packages.
The packaging repo is separate from the upstream repo (this one) and it can't have its own .gitlab-ci.yml file because its source tree needs to match our releases.
We can have our repo include another .yml file, if it exists. In this repo, it will build the Linux executables. In the packaging repo, a file can be added to enable it to also build the .deb packages.