See https://github.com/jenkinsci/docker/pull/668

I am using the base Dockerfile to build my own Jenkins instances, and it's difficult to update {{plugins.txt}}manually. Every time you need to launch the update center and then to go through the updates list and to update the file. And then rebuild the image until the UI is fine...

In order to simplify the use-case, I have added printing of available updates after the build (see availableUpdates). It will use the same update center as the installation logic, so the output will show only those updates which are actually applicable.

It would be great to have this functionality supported in the plugin management tool OOTB