Refactor main

[eunomie]

Move main.go to subfolder cmd/docker-index and update build steps.
The idea here is just to follow a quite common pattern where cmd/<something> produces the binary <something>. It's not mandatory at all, but might help to keep the code base organised.

✅ To be rebased once #11 is merged

[cdupuis]

Thanks for this. I didn't know this has emerged as a standard now.

[eunomie]

I didn't know this has emerged as a standard now.

Well... this is not really a standard. I think there's really different opinions on it. I prefer that way so it feels better organised from my views. And if at some point we need a different entry point then it's easy to add one. It's a bit of anticipation, but I don't really see downsides for now.
As an example, it's organised that way for gh CLI: https://github.com/cli/cli/tree/trunk/cmd