a. Code quality - tackled at code review time, and probably not dependent on python 2.7/3.0 differences; if they are version dependent then again it's a code review process. b. Code security - again, code review
Additional:
c. Third party dependency vulnerabilities: detection should be automated; github now supports automatic checks that open pull requests for vulnerable artifacts.
d. user acceptance tests - seems unlikely that two versions of python are used for the same part of the system.
Docker (especially docker-compose) makes it more visible as to what will be deployed. In fact, it may actually be better for security when those security checks of a system can be made before it goes to production, because the whole stack can be built, audited and executed locally.