Fix issues with Podman and S6 overlay

[PromoFaux]

What does this PR aim to accomplish?:

Should fix : #1176

TL;DR:

Before I updated the version of s6-overlay in the container, there was this line that moved /init to /s6-init, and the entrypoint was set to /s6-init.

I've just tested reverting this change (here: 18360d1), and pushed a new image with the :exp tag

Can you folk that are running into issues with podman please try pihole/pihole:exp and report back? Thanks!

(From a very brief test on my UDM running podman, it at least starts the container)

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