Update TESTING.md with docker test info

Signed-off-by: Derek Nola <[email protected]>

tests/TESTING.md

@@ -3,6 +3,7 @@
 Testing in K3s comes in 5 forms: 
 - [Unit](#unit-tests)
 - [Integration](#integration-tests)
+- [Docker](#docker-tests)
 - [Smoke](#smoke-tests)
 - [Performance](#performance)
 - [End-to-End (E2E)](#end-to-end-e2e-tests)
@@ -59,6 +60,12 @@ See [integration/README.md](./integration/README.md) for more info.
 
 ___
 
+## Docker Tests
+
+Docker tests run clusters of K3s nodes as containers and test basic functionality. These tests are run in the Drone CI pipeline `test` stage.
+
+___
+
 ## Smoke Tests
 
 Smoke tests are a collection of tests defined under the [tests](../tests) path at the root of this repository.