Pre-built Docker image

[nicola-corbellini]

Description

This PR updates the workflow to build and push a built docker image of the core.
You can keep this open and use it as a reference when we're ready to do this step.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas