Fix multi-platform builds with multiple aliases.

[zarthross]

Multi-platform builds append a -t argument for each alias to the dockerBuildOptions, but the publishTask function was ALWAYS iterating over the aliases even if it was a multi-platform build.

sbt-native-packager/src/main/scala/com/typesafe/sbt/packager/docker/DockerPlugin.scala

Lines 157 to 159 in 7c68859

	dockerBuildOptions := Seq("--force-rm") ++ dockerAliases.value.flatMap { alias =>
	Seq("-t", alias.toString)
	} ++ { if (dockerBuildInit.value) List("--init") else Nil },

The consequence was attempts to buildx and push the tags multiple times. In some environments this leads to build failures because you can't push the same tag twice.

This change simply does not iterate over the aliases for multiplatform builds.

Note: using dockerUpdateLatest := true counts as having multiple aliases, which is how I encountered this bug.

[lightbend-cla-validator]

Hi @zarthross,

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

We see that you have signed the Akka Contributors License Agreement before, however, the CLA has changed since you last signed it.
Please review the new CLA and sign it before we proceed with reviewing this pull request:

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

[zarthross]

The CLA signer process is broken... I cannot sign it... not sure what ya'll want to do about it.

[zarthross]

This is basically a 1 line fix, i do not care about the CLA, i do not care about copy right to the code, just wanted others to have the fix.....

[dwickern]

The CLA bot has an insecure http: URL... This link should work: https://www.lightbend.com/contribute/cla