[Grails 3] local images defined in 'docs' are not resolved

After migration to Grails 3 our Project Documentation generated using `docs` command has all local images references broken.

Steps done for demo project:
- Add grails-doc plugin to build: `apply plugin: "org.grails.grails-doc"`
- Add `application.groovy` file with `grails.doc.images = new File("src/docs/images")` option
- create `src/docs/images` and `src/docs/guide` folders
- add image to images
- add toc.yml to guide `page: Page`
- page.gdoc to guide `Image should be displayed !gr8confeu2016.png!`
- execute `gradle docs`

Documentation generated under `build/docs/manual` will look like:
Link to the image will be `src="../img/gr8confeu2016.png"`

But manual/img folder contains only following files:

Here is an example project with single page and image:
https://github.com/droggo/DocImagesIssue

Reproduce the issue in the linked DocImagesIssue project by running `gradle docs` with the `src/docs/images` and `src/docs/guide` inputs described here. Compare the generated `build/docs/manual/img` contents with the image referenced by `page.gdoc`; done means local images are copied or resolved so the generated manual displays them.

Written by the indexing model from the issue text.