Add docker tag to docker-tests target

Makes it easier to filter out this target where docker isn't installed.

Change-Id: I2c5906440221fbbbdbe313fe4024ea4e9b1c35f2

BUILD

diff --git a/BUILD b/BUILD
index 4843927..df9b631 100644
--- a/BUILD
+++ b/BUILD
@@ -43,4 +43,5 @@
     ],
     data = [plugin_name] + glob(["test/**"]),
     local = True,
+    tags = ["docker"],
 )