Support `make UPDATE_LATEST=false publish`
Support `make UPDATE_LATEST=false publish`

When publishing pre-releases via the release script we do not want to
modify the `latest` tag in DockerHub - specifying UPDATE_LATEST=false
causes publish to push to $WEAVE_VERSION only. Omitting the flag
entirely retains the existing behaviour for backwards compatability.

bboreham/weave,weaveworks/weave,awanke/weave,rade/weave,narry/weave,n054/weave,narry/weave,tomwilkie/weave,aaam/weave,n054/weave,narry/weave,beni55/weave,bboreham/weave,narry/weave,dpw/weave,aaam/weave,awh/weave,paulbellamy/weave,weaveworks/weave,n054/weave,beni55/weave,aaam/weave,weaveworks/weave,awanke/weave,narry/weave,aaam/weave,awanke/weave,n054/weave,awanke/weave,rade/weave,awh/weave,dpw/weave,squaremo/weave,philipz/weave,beni55/weave,dpw/weave,awh/weave,aaam/weave,awh/weave,bboreham/weave,awh/weave,squaremo/weave,squaremo/weave,awanke/weave,philipz/weave,weaveworks/weave,tomwilkie/weave,squaremo/weave,paulbellamy/weave,tomwilkie/weave,beni55/weave,rade/weave,beni55/weave,rade/weave,bboreham/weave,paulbellamy/weave,bboreham/weave,philipz/weave,paulbellamy/weave,philipz/weave,philipz/weave,dpw/weave,tomwilkie/weave,n054/weave,rade/weave,dpw/weave,squaremo/weave,tomwilkie/weave