Docker: improve tags support

Hello,

I noticed an issue when using tags other that latest, a few things stop to work like docker.pulled as it is using inspect_image behind the scene and inspect_image only gets 'name' as a parameter and not name + ':' + tag.
so docker.pulled always think it has to download the image as the hash is not found.
So i think we should have better support for tags and set the default value to 'latest' as it is the convention with docker.