Default docker image when using the form to create a competition

[Didayolo]

The following PR reversed a recent change:

• Fix adding queue and docker image #935

Adding value="codalab/codalab-legacy:py37" broke the editor: when saving a competition with a custom queue, the docker image was reset to the default one.

The goal of this change was simply to have codalab/codalab-legacy:py37 by default in the docker image field when creating a competition using the form (the creator). It should not have incidence on the editor. I create this issue to keep track of these little change we wanted to make.

(@bbearce)

[Didayolo]

This fix makes the manual competition creation test to fail:

https://github.com/codalab/codabench/blob/develop/src/tests/functional/test_competitions.py

https://app.circleci.com/pipelines/github/codalab/codabench/1241/workflows/850acb14-1325-4696-addc-d69ccaee6075/jobs/2965

=========================== short test summary info ============================
FAILED src/tests/functional/test_competitions.py::TestCompetitions::test_manual_competition_creation
============= 1 failed, 8 passed, 8 warnings in 292.95s (0:04:52) ==============

Maybe the manual competition creation test relies on the use of the default docker image?

Strangely, the tests did not fail on develop.