corrected syntax

Jenkinsfile

@@ -17,9 +17,9 @@ pipeline{
     	    steps{
      			// Shell build step
                 sh """ 
-                docker build . -t $DOCKER_REGISTRY/$DOCKER_USER/$JOB_NAME:$BRANCH_NAME --pull
+                docker build . -t $DOCKER_REGISTRY/$DOCKER_USER/$JOB_NAME:${env.BRANCH_NAME} --pull
                 docker login $DOCKER_REGISTRY/$DOCKER_USER -u $DOCKER_USER -p $DOCKER_PASSWORD
-                docker push $DOCKER_REGISTRY/$DOCKER_USER/$JOB_NAME:$BRANCH_NAME
+                docker push $DOCKER_REGISTRY/$DOCKER_USER/$JOB_NAME:${env.BRANCH_NAME}
                 """ 
     	    }
     	}