jenkins: www: Only rebuild docker image when Dockerfile was changed

* And use src directly

Change-Id: I0bf4246c7111347dd6dae463b010cd0bfbd6e481
diff --git a/jenkins/www/leaf-www-preview.groovy b/jenkins/www/leaf-www-preview.groovy
index dcff846..ff0f98e 100644
--- a/jenkins/www/leaf-www-preview.groovy
+++ b/jenkins/www/leaf-www-preview.groovy
@@ -32,9 +32,10 @@
                         export MYSQL_PWD="leaf"
                         echo "CREATE DATABASE leaf_ota; GRANT ALL ON leaf_ota.* TO 'leaf'@'localhost' IDENTIFIED BY 'leaf'; USE leaf_ota;" > build/leaf_ota.sql
                         mariadb-dump -h $MASTER_IP -u leaf leaf_ota >> build/leaf_ota.sql
+                        echo "${GERRIT_CHANGE_NUMBER}" > build/gerrit_change.txt
 
-                        docker build -t leafos/leaf_www --build-arg baseurl=${GERRIT_CHANGE_NUMBER} .
-                        docker run --rm -tv $(pwd)/build:/src/build leafos/leaf_www
+                        docker build -t leafos/leaf_www .
+                        docker run --rm -t -v $(pwd):/src leafos/leaf_www
                         if [ $? == 0 ]; then
                                 ssh -p 29418 LeafOS-Jenkins@review.leafos.org gerrit review -n OWNER --tag Jenkins --label Verified=+1 -m \\'"PASS: Jenkins : ${BUILD_URL}console\nBuild successful for change $GERRIT_CHANGE_NUMBER, patchset $GERRIT_PATCHSET_NUMBER.\nPreview available at https://www-preview.leafos.org/${GERRIT_CHANGE_NUMBER}"\\' $GERRIT_CHANGE_NUMBER,$GERRIT_PATCHSET_NUMBER
                         else