Merge branch 'patch-28' into 'master'

Update using_docker_build.md

See merge request gitlab-org/gitlab-ce!21927

doc/ci/docker/using_docker_build.md

@@ -314,8 +314,8 @@ build:
   stage: build
   script:
     - docker pull $CONTAINER_IMAGE:latest || true
-    - docker build --cache-from $CONTAINER_IMAGE:latest --tag $CONTAINER_IMAGE:$CI_BUILD_REF --tag $CONTAINER_IMAGE:latest .
-    - docker push $CONTAINER_IMAGE:$CI_BUILD_REF
+    - docker build --cache-from $CONTAINER_IMAGE:latest --tag $CONTAINER_IMAGE:$CI_COMMIT_SHA --tag $CONTAINER_IMAGE:latest .
+    - docker push $CONTAINER_IMAGE:$CI_COMMIT_SHA
     - docker push $CONTAINER_IMAGE:latest
 ```