Allow jumping to a Dockerfile from a Compose file's `dockerfile` attribute inside the `build` specification

[rcjsuen]

We should include a result for jumping to ../backend.Dockerfile when responding to a textDocument/documentLink request.

services:
  frontend:
    image: example/webapp
    build: ./webapp

  backend:
    image: example/database
    build:
      context: backend
      dockerfile: ../backend.Dockerfile

  custom:
    build: ~/custom