--pull does not propagate to docker build command

[staffanf]

Hi @avirshup!

I'm using DockerMake with the --pull option thinking that this would make it pull the base image manifest every time.
After looking at the code, I realize that the --pull option is not sent to the docker build command, it only affects sourceimages.
I.e. --pull option does not pull new FROM images if they have changed, this make it hard for us to have updated base images.

Changing line 113 in builds.py to step.build(client, usecache=usecache, pull=True) fixes this for me.

Is this is bug or is there a reason pull not being propagated?