= Jenkins setup for running meta-updater CI

As bitbake is quite resource-hungry, there are some special steps that are
needed to run Jenkins CI tasks:

- docker should be installed and the `jenkins` unix user should belong to
  the `docker` group
- `/opt/jenkins` should exist and have `jenkins:jenkins` permissions, it
  will be mapped as a volume on the same location in the docker build
  container

Note that for nodes running Jenkins slaves as a docker container, the
`/opt/jenkins` directory must exist on the host system as well, with
permissions matching the user and groupd ids in Jenkins' docker