Provide output feedback when building images

[davesann]

Building images can take some time. Progress output would be a good thing.

[hugoduncan]

Indeed. Will have to check if clj-http provides a mechanism for this.

[davesann]

yes I thought that one could be more tricky. I also found that the build is fast once the base images are available. Perhaps just recommending a docker pull of your base image before building is enough.