Update to Node v12 (LTS)

[mbsimonovic]

12 just became active LTS a few days ago, would be great to have this image updated.

circleci has also published version 12:
https://hub.docker.com/layers/circleci/node/12/images/sha256-50b91733b2d80088bd822929e3ff67373b2a04deebf2e0c5b51f9f6e031bb696