Revert 4/alpine to Node 12, Alpine 3.12
Revert 4/alpine to Node 12, Alpine 3.12

refs https://github.com/docker-library/ghost/issues/256
- node:14-alpine3.13 has an issue where the underlying sharp image library causes a segfault
- node:12-alpine3.12 doesn't appear to have this problem, so we'll use it for now

infosiftr/ghost,docker-library/ghost