I am building via cmake and I need access to some cmake module that is on the host filesystem, how should I expose those files to flatpak build ?
Is there some concept of mount (read only) a host file system during the build process
It might be possible in theory but you should never do this! Flatpaks are a sandbox and it’s goal is to work everywhere the same, including the build. This will of course not work, when something from your host system is used. It will also not possible to publish the App on Flathub. Just include the Modules in your Manifest.
you can’t.
Simply build the dependency part of the flatpak if its not in the SDK.