Update sample template for docker setup hook.

[morgoth]

"kamal" network is already created (in v2.0) so the sample code is no longer accurate.

Replacing with simple echo statement, as in few other samples.

If anyone has a real case sample that we could put in the file instead, then let me know. cc @tsvallender as you're the author (https://github.com/basecamp/kamal/pull/585)

[nickhammond]

What about a kamal server exec "docker version"?

[dhh]

Happy to take a more realistic example too. Not sure docker version is it.