ci-dockerfiles-sanity-check: Rework DOCKER_SUFFIX passing.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I8c541f835c85122db076a143e4addba587ea3bc4
diff --git a/ci-dockerfiles-sanity-check.yaml b/ci-dockerfiles-sanity-check.yaml
index 0e745a3..62302b6 100644
--- a/ci-dockerfiles-sanity-check.yaml
+++ b/ci-dockerfiles-sanity-check.yaml
@@ -22,6 +22,12 @@
     node: build-amd64
     concurrent: false
     display-name: 'CI Dockerfiles sanity check'
+    parameters:
+        - string:
+            name: DOCKER_SUFFIX
+            default: '-check'
+            description: |
+              Suffix for docker image name, should be non-empty to differentiate testing images from production images.
     scm:
         - git:
             url: https://review.trustedfirmware.org/${GERRIT_PROJECT}
@@ -63,8 +69,6 @@
         - build-name-setter:
             name: 'version.txt'
             file: true
-        - shell: |
-            export DOCKER_SUFFIX="-check"
         - shell:
             !include-raw: ci-dockerfiles-deployment/builders.sh
         - shell: