This will fix an issue with certain pylint issues when imported in
SonarQube. See https://community.sonarsource.com/t/illegalargumentexception-during-sonarscanner-execution-for-multiline-python-imports/70315.

With https://github.com/python-jsonschema/check-jsonschema/issues/112
check-jsonschema now works with !reference tags.

Otherwise conda cannot be found by GitLab CI.

The --cleanup flag is necessary to build multiple images in one
container. But that flag removes everything that's not in
the vanilla kaniko image.