[Feature Request] Add support for Dockerfile

The `docker-workflow-plugin` supports the use of a Dockerfile in the declarative pipeline script. This plugin (`docker-plugin`) seems to require images be pre-built and pre-defined via the UI (/configure > Cloud > Docker > Docker Agent template). Additional it means someone has to re-build the UI defined images manually (or create a scripted / automated process around it).

This is unfortunate as it removes a lot of the "infrastructure as code" ability by forcing admins to define and maintain the images in the UI instead of in a Dockerfile in the source repository of a Jenkins job or similar (like `docker-workflow-plugin`) which checks / builds the specified docker image automatically on the fly each time the job runs.

At minimum it would be nice to have a method to define a Dockerfile to use for each image defined in the Cloud > Docker UI. Better still would be some kind of direct support for Dockerfile in the declarative pipeline script.

Some of this is solved via the dockerNode syntax introduced for declarative pipelines in https://github.com/jenkinsci/docker-plugin/pull/681 but not using a Dockerfile from the SCM repo, I believe.