Update NodeJS to 8.12.x across our build images

8.12 is the latest NodeJS LTS, and is currently in use by the gitlab-ce/gitlab-ee CI images when building the assets.

We should ensure omnibus builds and others are updated to this version.

This is required for this upstream MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21825 where eslint is being upgraded. On our current versions of NodeJS and yarn, we were seeing the following errors when compiling that MR:

error eslint@5.6.0: The engine "node" is incompatible with this module. Expected version "^6.14.0 || ^8.10.0 || >=9.10.0".
error Found incompatible module

(Internal slack convo: https://gitlab.slack.com/archives/C0GQHHPGW/p1537432524000100)