Dockerfile: Upgrade Node from v10.x to v12.x

[DeeDeeG]

Context

• NodeJS promoted the 12.x branch to being a Long Term Service (LTS) release on October 21st, per their release schedule.
• Heroku automatically installs the latest (highest-numbered) LTS release unless we specify otherwise in our package.json (similar to how we have specified a version of Yarn.)
  • We should probably develop against the same version of Node as will be used in production. So we can pin a version in package.json, or just keep track and update the Dockerfile as needed.

Summary of Changes

• Switch from Node 10.x to 12.x in Dockerfile

Checklist

• [ ] Tested Mobile Responsiveness
• [ ] Added Unit Tests
• [ ] CI Passes
• [ ] Deploys to Heroku on test Correctly (Maintainers will handle)
• [ ] Added Documentation (Service and Code when required)

[DeeDeeG]

This only directly affects us in testing/ development, not in production, so it could be merged any time, before/after deploying etc. Not very time sensitive.

Also: The fact that this PR's CI tests pass, means that develop branch works fine under Node v12.

[DeeDeeG]

That said, I'll plan to merge this on the weekend,