I have a jenkins instance (which actually runs inside docker) for my Continous Integration.
The jenkins server builds docker images on an external docker host, tests
I'd add the below to other answers:
app.tag(["tag1","latest"])
Image.tag([tagname])
Runs docker tag to record a tag of this image (defaulting to the tag it already has). Will rewrite an existing tag if one exists.
From Jenkins docker step docs, at https://your/jenkins/instance/URL/pipeline-syntax/globals#docker