docs: Add guide for running Gitlab CI locally
docs: Add guide for running Gitlab CI locally

It's possible to use `gitlab-runner exec` to locally run the CI builds
as specified in `.gitlab-ci.yml`, without needing to copy over the
commands it runs. The run takes place inside the same Docker container
as the real Gitlab CI jobs.

BRANCH=None
BUG=None
TEST=(Ran commands in doc)

Signed-off-by: Tristan Honscheid <1e17951ff5ffc8afe254954c03de64afef62fe52@google.com>
Change-Id: Ief6d9f3add885eba0d6814c02602bc0f58d56797
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3840657
Reviewed-by: Jeremy Bettis <4df7b5147fee087dca33c181f288ee7dbf56e022@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec