quadlet: Support Type=oneshot container files

These just run once and are considered successful at exist. Not much is
needed to support it, but we have to avoid overwriting the type
with Type=notify.

Signed-off-by: Alexander Larsson <alexl@redhat.com>

test/e2e/quadlet_test.go

@@ -366,6 +366,7 @@ var _ = Describe("quadlet system generator", func() {
 		Entry("network.quadlet.container", "network.quadlet.container"),
 		Entry("noimage.container", "noimage.container"),
 		Entry("notify.container", "notify.container"),
+		Entry("oneshot.container", "oneshot.container"),
 		Entry("other-sections.container", "other-sections.container"),
 		Entry("podmanargs.container", "podmanargs.container"),
 		Entry("ports.container", "ports.container"),