Will the nightly tag for IA's Ignition Image be based on 8.3 now or will it continue to track 8.1 development?
If it does continue to track 8.1, will there ever be a new nightly tag for 8.3 so we can quickly test out new features as they're added?
The nightly will eventually switch to 8.3 but (not an informed statement, purely a guess) not until some time after 8.3.0 final is released.
For now we're planning ~weekly beta releases of 8.3.0.
@PGriffith nailed it, unsurprisingly. 
Further, we're planning to introduce an X.Y-nightly tag to help with your pinning needs between major versions.