Rename dockerTarget to dockerTag

[makubi]

Using SettingKey instead of TaskKey for the dockerTag setting.

The dockerTarget configuration is used as Docker tag and was not documented yet. Renaming it to a more applicable name.

[lightbend-cla-validator]

Hi @makubi,

Thank you for your contribution! We really value the time you've taken to put this together.

Before we proceed with reviewing this pull request, please sign the Typesafe Contributors License Agreement:

http://www.lightbend.com/contribute/cla

[muuki88]

Thanks. Looks good

[avakhrenev]

Hello @makubi! Why did you make dockerTag a SettingKey?

We are currently using 1.1.5 of native-packager, where we assign tags to images based on their content (hash). It allows us to skip building docker image and publishing it if image with the same tag already exists in the registry. So, our dockerTarget depends on docker:stage, which becomes impossible in 1.2.0.

Sorry if this is a wrong place to ask.

[makubi]

@avakhrenev I guess I just didn't think of a reason why to use a TaskKey here, but I guess you can change it again. What do you think @muuki88 ?