We did a lot of progress towards supporting read-only filesystems for Symfony app.
We managed to have the var/cache/ folder used almost only for build time artifacts.
I think we should do the last step and define the var/cache/ folder as the one used to store build time artifacts only.
There is at least one blocker: the cache pools, which need runtime write access.
Thus, I propose we add a new "tmp" folder, with a related $kernel->getTmpDir() method, defined as OK for storing runtime (and build time of course, for warmups) data.