Handle non-defined rpDockerPublish

[dwijnand]

In ff298c8 I tried to more accurately
model what I assumed was the intended behaviour, by wrapping the
invocation of rpDockerPublish in a Task. In doing so, however, I
dropped the .?, which originally allowed for rpDockerPublish to be
undefined. This caused a regression.

At this point I'm not sure why SbtReactiveAppPluginAll even exists. Why
not redefine publish in Docker in SbtReactiveAppPlugin, where
rpDockerPublish is actually defined? I went with the smaller change.

[eed3si9n]

Ref #19 "Only push when Docker plugin enabled and registry defined"

This redefines the publish in Docker task to only publish for tasks that enable the Docker plugin and define a registry.

This leads to a much better experience for Lagom projects where users will have the api projects and running docker:publish would lead to many error messages for them, as it would be falling back to the default publish implementation for the API projects which is likely to fail.

Ref #24 "Fix Cryptic code for shortcircuiting task"

Ref #143 "Clarify rpDockerPublish only if username & repo defined"

[ignasi35]

thanks @dwijnand for the quick response on fixing this one!

[eed3si9n]

I'm ok with merging for now, but we should remove the added magic on top of sbt-native-packager at some point.