ci: use docker buildx in travis

build/erpnext-nginx/Dockerfile

@@ -1,5 +1,6 @@
 ARG NODE_IMAGE_TAG=12-buster-slim
 ARG GIT_BRANCH=develop
+ARG DOCKER_REGISTRY_PREFIX=frappe
 FROM node:${NODE_IMAGE_TAG}
 
 ARG GIT_BRANCH
@@ -12,7 +13,7 @@ RUN chmod +x /install_app && \
 
 RUN /install_app erpnext https://github.com/frappe/erpnext ${GIT_BRANCH}
 
-FROM frappe/frappe-nginx:${GIT_BRANCH}
+FROM ${DOCKER_REGISTRY_PREFIX}/frappe-nginx:${GIT_BRANCH}
 
 COPY --from=0 /home/frappe/frappe-bench/sites/ /var/www/html/
 COPY --from=0 /rsync /rsync