Examples out of sync with release

[jottinger]

On the master branch, the example app shows the use of expose:

distDocker {
    dockerfile {
        expose 8080
    }
    dryRun = true
}

And the ocker plugin version is 1.3-SNAPSHOT, held locally. Looking at the source, I don't see where dockerfile is exposed at all in DockerPluginExtension.groovy at all - this makes the example not very useful at all, since I can't even do a deploy of the snapshot and get the functionality if the source doesn't support it. (Maybe it does? I can't tell how, if it does.)