Here's the magic incantation:
config.assets.cache_store = :null_store # Disables the Asset cache
config.sass.cache = false # Disable the SASS compiler cache
The asset pipeline has it's own instance of a cache and setting config.assets.cache = false does nothing, so you have to set its cache to be the null_store to disable it.
Even then, the SASS compiler has it's own cache, and if you need to disable it, you have to disable it separately.