Add a flag to pull the base image when building docker images

[jduan]

This is more of a request than an issue.

Right now, the plugin doesn't seem to pull the base image if it's always cached locally. This works for base images with specific versions but not base images with "latest" version. For "latest", I want the ability to pull it before building docker images.

It seems we can add a boolean flag (say alwaysPull) such that when the flag is true we would pass a "--pull" flag to this line: https://github.com/Transmode/gradle-docker/blob/master/src/main/groovy/se/transmode/gradle/plugins/docker/client/NativeDockerClient.groovy#L32

What do you think?