Document the CI setup

Signed-off-by: Dave Tucker <dt@docker.com>

docs/testing.md

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+LinuxKit Testing and CI
+=======================
+
+## Testing
+
+The test suite uses [`rtf`](https://github.com/linuxkit/rtf)
+To install this you should use `make bin/rtf && make install`.
+
+### Running the tests
+
+To run the test suite:
+
+```
+cd test
+rtf -x run
+```
+
+This will run the tests and put the results in a the `_results` directory!
+
+Run control is handled using labels and with pattern matching.
+To run add a label you may use:
+
+```
+rtf -x -l slow run
+```
+
+To run tests that match the pattern `linuxkit.examples` you would use the following command:
+
+```
+rtf -x run linuxkit.examples
+```
+
+### Writing new tests
+
+To add a new test, you should first decide which group it should be added to.
+Check the `test/cases` folder for a list of existing groups.
+Groups can be nested and there examples of this in the existing set of test cases.
+
+If you feel that a new group is warranted you can create one by `mkdir 000_name` where
+`000` is the desired order and `name` is the name you would like to use
+You must copy an existing `group.sh` in to this folder and adjust as required or you may use the
+[example](https://github.com/linuxkit/rtf/tree/master/etc/templates/group.sh)
+
+To write your test, create a folder within the group using the `000_name` format as described aboce.
+You should then copy an existing `test.sh` in to this directory and amdend it,
+or start from an [example](http://github.com/linuxkit/rtf/tree/master/etc/templates/test.sh)
+
+If your test can only be run when certain conditions are met, you should consider adding a label to
+avoid it being run by default and document the use of the label in `tests/README.md`
+
+## Continuous Integration
+
+The LiunxKit CI system uses [DatakitCI](https://github.com/moby/datakit/tree/master/ci)
+The configuration can be found [here](https://github.com/linuxkit/linuxkit-ci)
+
+## PR Testing
+
+Each PR is tested on disposable VM's spawned in Google Cloud Platform
+This machine has no privileges or credentials to talk to GCP or other cloud platforms.
+
+TODO: Add instructions on how to build a base image for LinuxKit CI in GCP.
+
+LinuxKit CI runs `make ci-pr` in the VM.
+This target runs the tests using `rtf` and the results directory is SCP'd back to the controller.
+The test results will be stored in DataKit for additional access
+Additionally a the `./artifacts` folder is SCP'd back to the controller.
+
+If the tests passed the next step is to check if the kernel config test image runs on GCP.
+The `./artifacts/test.img.tar.gz` file is used to create a GCP image by the Python scripts
+that are part of LiunxKit CI. It runs this image and greps for the success message in the logs.
+
+## Branch and Tag testing
+
+Branches and Tags are tested on a dedicated machine that runs in GCP.
+
+LinuxKit CI runs `make ci` or `make ci-tag` in the VM
+This target runs the tests using `rtf` and the results directory is SCP'd back to the controller.
+The test results will be stored in DataKit.
+
+If the tests pass, the GCP test is run in the same manner as described for PR tests.
+
+Finally, CI will run `make test-ltp` which will run the Linux Testing Project tests.