Spoiler: It did not go well.
SwiftUI previews already require a blood sacrifice. Adding Docker to the mix was like putting a hat on a hat — except the hat is on fire and the other hat is also on fire.
What I learned:
Xcode in a container is technically possible
Technically possible and "worth the effort" are very different zip codes
My Mac's fans have never been so opinionated
Current status: Back to building natively. My sanity thanks me.
Side note: If you've successfully run iOS builds in CI with containerization, please teach me your ways. I'll trade you my SwiftUI preview workflow (it works 60% of the time, every time).
apple containerization like https://github.com/Bob-dragon/containerization