For the moment this is just an experimental additional sidecar package
build process, separate from the one we really use for releases, so that
we can get some experience building in the GitHub Actions environment
before hopefully eventually switching to using the artifacts from this
process as the packages we'll release through the official release
channels.

It will react to any push to one of our release branches or to a release
tag by building official-release-like .zip, .deb, and .rpm packages, along
with Docker images, based on the content of the corresponding commit.

For the moment this doesn't actually produce _shippable_ packages because
in particular it doesn't know how to update our version/version.go file
to hard-code the correct version number. Once Go 1.18 is release and we've
upgraded to it we'll switch to using debug.ReadBuildInfo to determine
our version number at runtime and so no longer need to directly update
a source file for each release, but that functionality isn't yet available
in our current Go 1.17 release.