Docker Swarm Mode has been in severely declining usage and maintenance for years now. Meanwhile, Docker Compose has had quite a lot of changes (notably deprecating the `version:` field required by Swarm / forward compatibility). As a result, it no longer makes a ton of sense for us to prefer/suggest Swarm usages.
This changes all references to be `compose.yaml` (the upstream-preferred canonical filename: 9a9cc5d9c3/cli/options.go (L384-L385)) directly, leaving "swarm compatibility" as an exercise for the minority of readers who might still need/want it.
Unfortunately, this also means we need to remove all the play-with-docker links (since it only supports stack deploy via URL arguments, not compose), but my experience with that service is that it hasn't been terribly performant for a while, so I don't think this is a huge loss.
(There were also a few trailing references to the long-since-defunct Docker Machine project which have also been cleaned up.)
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