Add latest branch tag for gitlab-qa images

What does this MR do and why?

Describe in detail what your merge request does and why.

Additionally tags gitlab-qa docker images with latest branch tag. Useful when needing to run e2e tests with latest code from master or any other branch which is not easily done currently as images are only tagged with specific commit shas.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.