[Node.js][Docker] Support node versioning within Dockerfiles

[stefafafan]

Regarding Node.js, Renovate updates the version up to the latest LTS (e.g. currently the latest is 16.18.0, so if you have v12, the updates will be v14, v16, and stop).

However, when the node version is within the Dockerfile like FROM node:12-alpine , Renovate does not treat it as the same as .node-version file, and just updates it up to the latest (e.g. to version 18).

I've seen this discussion here https://github.com/renovatebot/renovate/discussions/14909 and thought this should be useful for the other teams as well and tried to add a config file for this.

What do you think? @ikesyo ?

Here is a Japanese entry from me. Renovate で Dockerfile 内の Node.js のバージョンも LTS版まであげてもらう - stefafafan の fa は3つです