Use --pull option when building images

[nuest]

In

https://github.com/o2r-project/o2r-muncher/blob/master/lib/executor.js#L830

enable the --pull option to attempt to pull a newer version of base images, see https://docs.docker.com/engine/reference/commandline/build/

Potentially it might be more efficient in the big picture to pull base images regularly... for productive deployments probably a local hub where this pull is very fast is best.