Allow image version to be passed in image name line

Regarding the post :

https://community.atlassian.com/t5/Bitbucket-Pipelines-questions/Repo-env-var-in-image-line/qaq-p/1046325

I think is would be very good for maintaining multiple projects that use the same hosted image, for the version to set and passed via a repo environment variable.

To me, this makes sense, as you don't always want to use :lastest as after an image has been updated you probably want to manually update those projects you want updating, and leave those you don't as they are.