Unnecessary resources' content

As I can see resources directory content is being added and removed. I assume it's because when it's content is removed build fails as it cannot generate .css files.

I would like to propose the following changes:

1. change Hugo version in image used to build a site to registry.gitlab.com/pages/hugo/hugo_extended
2. merge docker branch to master for better visibility

Context of the proposition:

ad 1. Working on my fork of this repository I found out that it is enough to use extended Hugo image to prevent described failures. While it may seem to be a minor inconvenience it was a big problem for me, as I tried to generate a site (for review purposes) with automatically generated baseURL. Resources' structure is based on baseURL and it forced me to use the extended image to be able to generate it during a build.

ad 2. Figuring out which version of Hugo is used to build site in GitLab's pipelines took me way too much time as it was not clear where to find docker's definition. I hope it will prevent more people from losing their time looking for it :)