gitlab: don't run CI jobs in forks by default

gitlab: don't run CI jobs in forks by default

To preserve CI shared runner credits we don't want to run
pipelines on every push.

This sets up the config so that pipelines are never created
for contributors by default. To override this the QEMU_CI
variable can be set to a non-zero value. If set to 1, the
pipeline will be created but all jobs will remain manually
started. The contributor can selectively run jobs that they
care about. If set to 2, the pipeline will be created and
all jobs will immediately start.

This behavior can be controlled using push variables

  git push -o ci.variable=QEMU_CI=1

To make this more convenient define an alias

   git config --local alias.push-ci "push -o ci.variable=QEMU_CI=1"
   git config --local alias.push-ci-now "push -o ci.variable=QEMU_CI=2"

Which lets you run

  git push-ci

to create the pipeline, or

  git push-ci-now

to create and run the pipeline

Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20220526110705.59952-6-berrange@redhat.com>
[AJB: fix typo, replicate alias tips in ci.rst]
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20220527153603.887929-33-alex.bennee@linaro.org>