Add option to pull when building about docker-maven-plugin

[davidxia]

@phjardas What's your use case? I couldn't find the docs for the --pull. Do you have a link?

[phjardas]

We have Jenkins builds distributed over several slave servers. When one of our base images (tagged with :latest is updated in a job on one of the slaves, then other jobs that build on other slaves that use this base image should use the updated image.

Hence we want to always pull before we build.

Here's a link to the documentation: https://docs.docker.com/reference/commandline/build/

The option is --pull=false Always attempt to pull a newer version of the image

Cheers!

[davidxia]

@phjardas It depends on a change to its dependency docker-client. But I can't get this test to pass https://github.com/spotify/docker-client/pull/217/files#r36091657

[phjardas]

Hooray, thanks a lot!