Pinned golang and node to older versions in Dockerfile

[AndrewO]

Comparing release histories, I picked golang 1.18 and node 14 as likely candidates for the old build. I also moved to a multistage build so that the only thing left was the compiled binary.

I was running into the same issue as in #78 so I pinned Node to 14 (which, I believe was LTS when this was last released). Then I made it a multi-stage build container so that none of those older versions would be around in the final artifact.