Add version number for assets versioning

[EmilePerron]

Assets versioning in Koalati is handled via a version number stored in the KOALATI_RELEASE_VERSION environment variable.

At the moment, this version is not defined in the base images nor in the .env template of this template, which causes the default dev value to be used at all times.

So when an update is made, users must clear their browser cache to get the new assets. This can be problematic for scripts and styles.

This could likely be fixed by either defining an environment variable directly in the base image published to Docker Hub (in the workflow of koalatiapp/app.