test DIND under both arch runners

Signed-off-by: Leonardo Sandoval <leonardo.sandoval@linaro.org>

.gitlab-ci.yml

-test:
-  tags:
-    - arm64-runner
+build:
+  image: docker:dind
+  before_script:
+    - dockerd -l fatal &
   script:
-    - echo "hello world"
+    - docker info
+  parallel:
+    matrix:
+      - TAG: [arm64-runner, amd64-runner]
+  tags:
+    - $TAG
+