Use UID number rather than username in Dockerfile

Systems configured to disallow running images as root aren't able to run images that use user name string values for the `USER` because they can't validate that a named user isn't root. To allow this image to run on such systems, use the uid of the user as the value for `USER` instead of the username.

See: kubernetes/kubernetes#56503

docker/Dockerfile

@@ -18,7 +18,7 @@ COPY --from=build /pack/markdownlint-cli2-*.tgz /
 RUN npm install --global --no-package-lock --production /markdownlint-cli2-*.tgz
 RUN rm /markdownlint-cli2-*.tgz
 
-USER node
+USER 1000
 
 WORKDIR /workdir