Add support of packaging LibreCores CI as a WAR/Docker file using Custom WAR Packager

[oleg-nenashev]

Currently the hosted instance of LibreCores CI runs without Docker due to the Docker-in-Docker limitations I wanted to avoid at that point. Nowadays there is a Custom WAR Packager, which can package both WAR files and Docker images.

It would be interesting to adopt the Custom WAR Packager build flow in the repository so that we can deliver the patches to ci.librecores.org from this repo.

Acceptance criteria:

• There is a Jenkinsfile added to the repository
• ci.librecores.org is configured to build a job using the Jenkinsfile
• The self-build flow is offered as one of the demos in this repository