core (main): add a mechanism to have a shared temp directory: this directory may be used to share files between backend and frontends. Normally, an os-dependent temporary directory is created for that, but if this option is set, the directory will be created in <local_dir>/<cache_dir>, which may be useful in some use case (e.g. containerized frontends and backend).

This directory and subdirectories contain files to build and manage Docker images of Libervia.

Please refer to documentation for details.