Move some defaults of docker to global scope

[swsnr]

SBT strongly recommends to use the global scope for default settings, see https://www.scala-sbt.org/1.x/docs/Plugins-Best-Practices.html#Provide+default+values+in

This allows to set the docker image and common labels once in ThisBuild for all subprojects, and makes it easier to use a different docker-compatible container builder by overriding the command, e.g.

sbt 'set ThisBuild / dockerExecCommand := Seq("podman")' docker:publish

[lightbend-cla-validator]

Hi @lunaryorn,

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 Lightbend Contributors License Agreement:

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

[swsnr]

I should like to point out that while I am fairly confident that this works I didn't test it, because I'm admittedly too lazy to bother with publishing locally and adding it to a project :grimacing:

[muuki88]

1.8.1 is being released

[swsnr]

Thank you.