devtools: update node version in CI

The npm version that comes with node 14 is behind
the version we are using locally. Updates node to 15
to get an image with npm 7.x.x.

Change-Id: Ida4584693d86127ff145fde35146e51bda58da36
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/313029
Trust: Jamal Carvalho <jamal@golang.org>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

devtools/docker_nodejs.sh

@@ -8,4 +8,4 @@ set -e
 
 # Script for running a nodejs docker image.
 
-docker run --rm -v `pwd`:/pkgsite -w /pkgsite  node:14.15.1 $@
+docker run --rm -v `pwd`:/pkgsite -w /pkgsite  node:15.14.0 $@