[#1689] ci: Add commit checker to Jenkinsfile

- Commit checker image is built from dco-go:
  TrueCloudLab/dco-go#14
- 'pull_request_target' branch is defined in Jenkins job:
  TrueCloudLab/jenkins#10
  TrueCloudLab/jenkins#11

Change-Id: Ib86c5749f9e084d736b868240c4b47014b02ba8d
Signed-off-by: Vitaliy Potyarkin <v.potyarkin@yadro.com>

.ci/Jenkinsfile

@@ -78,6 +78,10 @@ async {
             }
         }
     }
-}
 
-// TODO: dco check
+    task('dco') {
+        container('git.frostfs.info/truecloudlab/commit-check:master') {
+            sh 'FROM=pull_request_target commit-check'
+        }
+    }
+}