Stabilize the DAST pipeline by pinning to a specific version of Docker

What does this MR do?

Fixes the DAST Pipeline by pinning to version 19.03.10 of the Docker image. For some reason, when using version 19.03.11, the installed version of python3 doesn't setup the pip3 command, breaking the DAST pipeline.

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
  • Job definition example
  • Vendored CI Templates (also in CE)
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer