Feature request: Allow volume to be passed with data to be imported

[guneemwelloeux]

The Firebase emulators (Auth, Firestore and Storage in particular) allow data to be saved to a local folder upon exit, and imported at startup. This allows one to re-start the emulators with some data already populated.

It would be great for the Dockerfile to allow a volume to be attached, and read data from there, as is done in this other image, for example: https://github.com/SpineEventEngine/gcp-emulators/tree/master/firebase-emulator#provide-emulators-baseline-data