[db] A CI job to run alembic migrations would be useful

Following up on #112 (more specifically https://github.com/pyronear/pyro-api/pull/112#discussion_r584346620), we need to automate the alembic migration whenever it needs to be run. Depending on the integration, Github Workflow could help with that! 