Use docker node image

validator/README.md

@@ -129,19 +129,18 @@ Now `cd amphtml/validator` and run `python build.py`.
 In the case the test passes in your local machine but fails in Circle CI,
 you can use docker to reproduce the test errors.
 
-1. Start an interacitve docker container
+1. Start an interactive docker container
     ```bash
-    docker run -it ubuntu bash
+    docker run -it node:lts-buster bash
     ```
 1. Run following commands in the container. Note that you are already the `root` user inside the docker container.
     ```bash
     apt update
-    apt install -y curl git gpg sudo
-    curl -fsSL https://deb.nodesource.com/setup_16.x | bash -
-    apt install -y nodejs
+    apt install -y sudo
     git clone https://github.com/ampproject/amphtml.git
     cd amphtml
     npm install
+    npm run postinstall
     .circleci/install_validator_dependencies.sh
     amp validator-cpp && echo SUCCESS || echo FAIL
     ```