Fix build command

Signed-off-by: Stefan Scherer <stefan.scherer@docker.com>

Jenkinsfile

@@ -10,7 +10,7 @@ pipeline {
             when {
                 branch 'master'
             }
-            sh "docker build docker/getting-started ."
+            sh "docker build -t docker/getting-started ."
 
             steps {
                 withDockerRegistry([url: "", credentialsId: "dockerbuildbot-index.docker.io"]) {
@@ -19,4 +19,4 @@ pipeline {
             }
         }
     }
-}
+}