https://github.com/jenkinsci/custom-war-packager/pull/51 introduced caching of plugins and non-plugins in local Maven repos when pom.xml is used as an input. Unfortunately such approach does not work well when builds are performed in Docker without connected Maven cache.

I propose to add support of external cache files, which could be used to warmup the initial cache in CWP and hence to speedup the execution.

Acceptance criteria:

• Custom WAR Packager can take lists of plugin and non-plugin artifact IDs. Format is TBD, but it may be enough to just pass 2 text files with lists OR a simple YAML file
• Custom WAR Packager CLI can generate cache files for the provided configurations
• There is a demo. It might be interesting to have a demo which demonstrates how to use intermediate caching as a multi-stage Docker build