run migrations automatically on backend startup (AUTO_MIGRATIONS)

Add a migrate.sh config (ported from lasuite-docs) that the backend
entrypoint runs when AUTO_MIGRATIONS=true (default). It loads secrets,
waits for the DB, and idempotently applies pending migrations via
'migrate --check' then 'migrate --noinput'. The manual 'abra app cmd
backend migrate' now delegates to the same script.

README.md

@@ -27,11 +27,12 @@ This recipe requires four domains. One domain for drive, and one for minio which
 * `abra app config <app-name>`
      - make sure to set MINIO_DOMAIN, COLLABORA_DOMAIN, ONLY_OFFICE_DOMAIN to the domains you set up for each. 
 * `abra app deploy <app-name>`
-* `abra app cmd <app-name> backend migrate` # creates database tables
 * `abra app restart <app-name> minio-createbuckets` (Note: this will appear to fail, but probably worked! Check `abra app logs <app-name> minio-createbuckets`)
 
 You should then be able to visit the landing page of your app, but not yet to login. To login, you need to deploy and integrate single sign on (described below in the "Configure Authentication" section). 
 
+* Migrations run automatically on backend startup. To trigger manually: `abra app cmd <app-name> backend migrate`
+
 Wopi discovery is supposed to happen automatically, but if collabora/onlyoffice are not connecting, you can try running:
 
 * `abra app cmd <app-name> backend trigger_wopi` # connects only office & collabora (if they stop working, try running this again)