Lets say you've built $BUILD_IMAGE container image at the build step. I did it on a NodeJS based project but it will work with other technology as well.

check-openapi-contract:
  stage: test
  retry: 1
  timeout: 15m
  script:
    - docker run --name=my-container -d -i -p 8080:8080 --rm $BUILD_IMAGE npm start
    - bash -c 'while [[ "$(curl -s -o /dev/null -w ''%{http_code}'' localhost:8080/swagger.json)" != "200" ]]; do sleep 5; done'
    - docker exec -i my-container curl http://localhost:8080/swagger.json -o ./swagger.json
    - docker exec -i my-container npx swagger-cli validate ./swagger.json

So what do we do? We start the server, then retrieve the swagger.json or openapi.json and leverage swagger-cli validate command to ensure our definition is valid and be notified if it is not. Nothing. More.