[Docker] add multiple docker tags support

[kimxogus]

Added additional docker image tags support.

Before, if your project's version is 1.1.0 and you want to tag 1.1 and 1, you had to tag 1.1 and 1 manually outside of sbt task. With this PR, you can specify additional docker image tags and manage them in sbt.

[muuki88]

Thanks again for your awesome work on the docker plugin 😍

The general implementation idea is great. Defining a container dockerAliases, which contains all aliases to be built. I would actually like to cut it down to this setting without introducing any another settings. The reason is that any new setting needs to be maintained (managed, documented, etc.)

I would suggest the following way:

1. Keep the dockerAlias setting as the "primary docker alias". We use this as the starting point for other aliases
2. Add helper methods on the DockerAlias class for a more readable API, e.g. withTag / withUntagged, withusername

For users this would look like this

dockerAliases += dockerAlias.value.withUntagged(),
dockerAliases += dockerAlias.value.withTag("foo")

WDYT?

[kimxogus]

That sounds much better. That would be more intuitive. I'll change them

[kimxogus]

• Updated DockerAlias with more readable member methods
• Updated sbt-scalafmt because it reformats differently with Intellij's scalafmt.

[muuki88]

Thanks a lot for the changes 🎉 This looks smooth an clean.

I'll merge once the build is green.

Also thanks for upgrading scalafmt. Normally I would like to have such changes in a separate PR to be able to squash and merge it more easily, but it's fine here as the changes are rather minimal.

Thanks for your time to make sbt-native-packager more awesome 😎

[dwijnand]

RIP binary API...

[muuki88]

Really time to add MiMa :(