Avoid docker pull on each container run

Currently the docker containerizer does a docker pull on each run, and this has several downsides:

1. Not able to run local images
2. Require to contact registry server on each run, therefore docker run becomes unavailable if the registry server is down. Also has scalability limits.

We want to avoid doing a pull everytime. The downside ofcourse is that images without explicit tags (:latest) will not get the most updated version, but this is the same behavior when calling docker run on the cli.