DockerPlugin publishTask: switch --push flag to --output=type=registry

[zachdecook]

--push is shorthand for --output=type=registry (https://docs.docker.com/engine/reference/commandline/buildx_build/#push)

By specifying --output=type=registry, sbt can support when docker is a symlink to podman.

[lightbend-cla-validator]

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[zachdecook]

This is incorrect. (it fixes a symptom of the problem, but podman requires a second command to be run to push it).

Probably it's best fixed by patching podman.