🐛 Bug: Improve versioning in Docker builds about ersilia

[miquelduranfrigola]

We decided:

1. Versioning Ersilia base image (and other images created with the corresponding GitHub Action in the ersilia CLI repo). Version should be in synch with the actual ersilia version. The latest tag should continue to exist.
2. Models will be versioned with a timestamp (e.g. 2024.03.18).
3. The version of the ersilia base image used for building the model will be implicit and can be queried at model building time. Importantly, the FROM ersiliaos/ersilia:latest used at model building time should be replaced to use the exact version of the ersilia base image. This should be done dynamically in the GitHub Action.
4. We should add a flag to the ersilia fetch command (e.g. --version 2024.03.18) in case users want to use an older version of the model.