Corrected reference to global option 'filter'
Corrected reference to global option 'filter'

The global option 'image' was renamed to 'filter' but the push section referenced the old name.
Signed-off-by: Tamsin Slinn <595c7a6ca842bae3caf03a5313500f1d8c1a4b1f@gmail.com>
vjuranek/docker-maven-plugin,scoplin/docker-maven-plugin,fabric8io/docker-maven-plugin,fabric8io/docker-maven-plugin,vjuranek/docker-maven-plugin,thomasvandoren/docker-maven-plugin,vjuranek/docker-maven-plugin,rhuss/docker-maven-plugin,mattbetzel/docker-maven-plugin,rhuss/docker-maven-plugin,fabric8io/docker-maven-plugin