I have a workable, but probably incomplete, answer to my question from the output of compiler.package.docker. If I copy those lines to a text file, it seems to work. Note that unlike the installer input file for regulat MATLAB, the Runtime one appears not to require equals between argument and value.
The bit I was most interested in was the product.<name> true line, which lets you install just the Runtime products you need. This is addressed in my other question.
Thanks to @Raymond Norris for suggesting I look at compiler.package.docker.
Edit: this works with MATLAB R2021a / Runtime v9.10, but not in R2019b. Selective installation of Runtime seems to be a recent addition.