Always pull the container image before attempting to create it

Docker will not automatically pull an image on container creation if the
image wasn't previously retrieved (this behavior differs from the
'docker run' command line). We need to make sure the container image is
available before creating the container by pulling the image beforehand.
Pulling an already retrieved image has no effect, so we can simply do it
unconditionally.

Signed-off-by: Maxime Petazzoni <max@signalfuse.com>