I encountered this issue on an upgrade of IAM, it wasn't covered in the documentation.

Kong is prompting to have the postgres DB structure updated to match the newer version of Kong. I solved it by changing the docker-compose.yml to contain the following config to run 'kong migrations up', then started the container:

  iam-migrations:
      image: ${ISC_IAM_IMAGE}
      command: kong migrations up
      depends_on:
        - db
      environment:
        KONG_DATABASE: postgres
        KONG_PG_DATABASE: ${KONG_PG_DATABASE:-iam}
        KONG_PG_HOST: db
        KONG_PG_PASSWORD: ${KONG_PG_PASSWORD:-iam}
        KONG_PG_USER: ${KONG_PG_USER:-iam}
        KONG_CASSANDRA_CONTACT_POINTS: db
        KONG_PLUGINS: bundled,jwt-crafter
        ISC_IRIS_URL: ${ISC_IRIS_URL}
      restart: on-failure
      links:
        - db:db