Steps to reproduce:

1. create a freestyle job that will be building a docker image
2. fill in source details (any repo with a Dockerfile)
3. add a Docker Builld and Publish step
4. fill in a repository name (e.g. myimg)
5. use a shortname of a custom registry in Docker registry URL field (e.g. https://myregistry:443/v1/
6. save and run the job

Expected outcome:
Job succeeds, the image is tagged as myregistry:443/myimg and pushed OK.

Actual outcome:
Job fails, the image is tagged as myregistry/myimg which means docker push tries to push it to the public DockerHub registry (and fails as there are no credentials).

Notes:
I suspect this is in DockerBuilder.Perform.getNameAndTag defined in https://github.com/jenkinsci/docker-build-publish-plugin/blob/master/src/main/java/com/cloudbees/dockerpublish/DockerBuilder.java#L272 or possibly even in DockerBuilder.getRepo. I'm not familiar with the code so wasn't sure what will be affected if that is changed.