Pull Image for Docker Runner

Issue Summary

It'd be nice if there was an option to forcefully pull an image for docker runner, so that when, say, latest tag is updated it's available on all agents. Currently, docker pull use an existing tag on a host and will not go to a remote.

Steps to Reproduce

1. Configure job to be executed in a docker image. Run the job
2. Update docker image, push to registry, re run the job. Bamboo will use an old tag

Workaround

Run docker pull tasks before the main task. See Docker Runner doesn't pull the latest image automatically