ci-dockerfiles-sanity-check: Reinstate matrix, use build-amd64-private

We can't convert marix job to normal job, that's apparently the reason
why there's as single-value matrix job in the first place.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: Iecc9705c9585a9866b334f9cb2b1f2a321e44be6
diff --git a/ci-dockerfiles-sanity-check.yaml b/ci-dockerfiles-sanity-check.yaml
index 845ab65..7ea35f3 100644
--- a/ci-dockerfiles-sanity-check.yaml
+++ b/ci-dockerfiles-sanity-check.yaml
@@ -65,12 +65,12 @@
                     - branch-compare-type: 'PLAIN'
                       branch-pattern: 'master'
             silent-start: false
-#    axes:
-#        - axis:
-#            type: slave
-#            name: label
-#            values:
-#                - build-amd64
+    axes:
+        - axis:
+            type: slave
+            name: label
+            values:
+                - build-amd64-private
     execution-strategy:
         sequential: false
     wrappers: