feat: happiness both on fly.io and in docker compose

[d-led]

Change Summary

What and Why:

Problem:

• when launching a fresh phoenix app that wasn't yet deployed in docker, sensible default files are generated
• the defaults are valid for fly.io but e.g. not for local docker compose set up, where there's no ipv6
• trying to get (auto-)clustering right might frustrate users with phoenix and elixir. Since developer happiness is writ large, this frustration might be averted
• I've fixed this before but forgot I did → re-work, same debugging session

How:

• generate a sensible env.sh.eex that will work both with libcluster in docker compose and with dns_cluster on fly.io

Related to:

• there are some defaults generated by phx.new that could be improved (will gladly follow up with the worked out patterns)

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Documentation

• Fresh Produce
• In superfly/docs, or asked for help from docs team
• n/a