For a source file like `foo.container`, look for drop in named
`foo.container.d/*.conf` and merged them into the main file. The
dropins are applied in alphabetical order, and files in earlier
diretories override later files with same name.
This is similar to how systemd dropins work, see:
https://www.freedesktop.org/software/systemd/man/latest/systemd.unit.html
Also adds some tests for these
Signed-off-by: Alexander Larsson <alexl@redhat.com>
Alexander Larsson