Allow PIXL DB tables to be updated in production

[stefpiatek]

Definition of Done / Acceptance Criteria

• instead of creating tables from sqlalchemy in postgres/create_pixl_tbls.py, use define the tables in alembic migrations and run them before we run the imaging-api/pacs-api server as part of the run command

Testing

shouldn't need any changing

Documentation

Add documentation on alembic being used for the postgres service

Dependencies

No response

Details and Comments

• We expect that the pixl DB will change over time, and want to handle this in an automated way that we can test with development versions
• Because we're defining our tables in sqlachemy, alembic is a straightforward choice. Its a bit of duplication of information, but does mean that we can update our production database in a controlled way with rollbacks if required
• the imaging pipeline is the only part that depends on the pipeline tables, so running this on startup of the container makes sense

[jstutters]

Underway in jstutters/270-use-alembic-for-database-table-creation.