Jobs based on custom runners: build environment docs and playbook

Jobs based on custom runners: build environment docs and playbook

To run basic jobs on custom runners, the environment needs to be
properly set up.  The most common requirement is having the right
packages installed.

The playbook introduced here covers the QEMU's project s390x and
aarch64 machines.  At the time this is being proposed, those machines
have already had this playbook applied to them.

Signed-off-by: Cleber Rosa <crosa@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210630012619.115262-3-crosa@redhat.com>
Message-Id: <20210709143005.1554-3-alex.bennee@linaro.org>