Motivation for this change
In this pull request, we provide Nix expression for evaluating concourse and fly, as well as systemd service units for setting up concourse ATC and/or worker.
Not sure what the problem is. In any case you should clean up the commits in this PR. This means:
* concourse: init at 4.3-unstable * WIP * add toplevl packages * make concourse build on linux * concourse: init at 4.3-unstable * WIP * add toplevl packages * packr * bump concourse version * service definition * change some defaults
* separate concourse and fly * move preBuild commands out of buildConcourse * different names for different derivations