The Docker engine itself, meaning the dockerd daemon, its APIs and the docker CLI, are all under Apache 2.0. The non-free parts are mostly in their Docker Desktop offering, which is mostly a convenience GUI and not absolutely necessary (the easiest, on Windows and macOS, probably, but not the only one) to run Docker on most platforms.

This. My old 2nd gen i5-750 doesn’t hold a candle vs. a modern i3.

When people say x86 is not power efficient, it usually means it’s not very efficient for battery powered devices, or is kind of wasteful in situations like in data centers where they’re running thousands of machines. For home use, with a machine that’s gonna probably end up idling most of the time, my best guess is it would cost you a couple tens of dollars a year to run vs a slightly smaller amount.

Personally, just so I don’t have to deal with software compatibility on different architectures, I’ll gladly pay that small difference in power usage, but this will of course vary depending on what you’re looking to run on there.