Cope with multiple processes get_remote_image_info in parallel (#9105)
Cope with multiple processes get_remote_image_info in parallel (#9105)

When I'd made a change to a large number of python files, running the
flake8 pre-commit hook would fail without obvious error (as in no error
was printed, but exit code was 1).

In debugging this I switch the pre-commit to `require_serial: true` and
the problem went way - the fix for this is:

- Don't redirect stderr to /dev/null (that silences both our VERBOSE
  trace output, and the errors from docker)
- Use `--cidfile` option to docker to create a random name and write the
  created container ID to a file
wooga/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,mistercrunch/airflow,dhuang/incubator-airflow,cfei18/incubator-airflow,bolkedebruin/airflow,danielvdende/incubator-airflow,mistercrunch/airflow,cfei18/incubator-airflow,nathanielvarona/airflow,airbnb/airflow,wooga/airflow,mistercrunch/airflow,DinoCow/airflow,cfei18/incubator-airflow,apache/incubator-airflow,wooga/airflow,mistercrunch/airflow,airbnb/airflow,airbnb/airflow,nathanielvarona/airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,airbnb/airflow,DinoCow/airflow,Acehaidrey/incubator-airflow,mrkm4ntr/incubator-airflow,apache/airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,wooga/airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,lyft/incubator-airflow,sekikn/incubator-airflow,lyft/incubator-airflow,dhuang/incubator-airflow,DinoCow/airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,apache/airflow,mrkm4ntr/incubator-airflow,sekikn/incubator-airflow,lyft/incubator-airflow,apache/airflow,bolkedebruin/airflow,dhuang/incubator-airflow,danielvdende/incubator-airflow,sekikn/incubator-airflow,cfei18/incubator-airflow,apache/airflow,mrkm4ntr/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,mrkm4ntr/incubator-airflow,dhuang/incubator-airflow,apache/airflow,nathanielvarona/airflow,apache/incubator-airflow,apache/incubator-airflow,danielvdende/incubator-airflow,sekikn/incubator-airflow,DinoCow/airflow,nathanielvarona/airflow,apache/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow