Make the image name configurable and get things working on the SteamDeck

[spkane]

I happened to be giving this a try on my SteamDeck and was able to get things working well with a few tweaks.

In general, this:

• Makes the image repo, image name, and image tag configurable.
  • Expands and adds a few scripts to facilitate the new flexibility.
• It also makes a few adjustments that allow this to work on a real SteamDeck.
  • Change the GUID on the image partition we want to mount, as it needs to be different then the real parition that is already mounted on the SteamDeck.
  • Use podman which is pre-installed on the SteamDeck.
  • Add some additional notes to the README.