SOLR-18032 Use SOLR_DOCKER_PLATFORM only for dockerPush command

[janhoy]

Modification to previously merged solution in #3964 that makes it not affect dockerBuild or testdocker, only dockerPush. It has little value to build or test on a different architecture than local. This simplifies the solution, and will also make it possible to run testDocker and dockerPush with multi platform in the same gradle command.

https://issues.apache.org/jira/browse/SOLR-18032

[janhoy]

Sorry for the back and forth @risdenk, but I want to simplify the solution from the PR yesterday, which tried to suport SOLR_DOCKER_PLATFORM for dockerBuild and testDocker. After seeing how Jenkins docker builds are setup, they run both testDocker and dockerPush tasks in the same Jenkins step, but that fails now since it complains that build is not supported for multi platform.

So I could either split gradle task in Jenkins into a separate testDocker and dockerPush task or rewrite it as done in this PR. I landed on this simple solution, as I have not so far seen an actual use case for building for multi-arch without also pushing. Such feature could be added (in a different way) later if actually needed.

PS: I plan to do CTR on this PR, and trigger a new Jenkins build..