Improve the pipelines documentation

Request

The explanation about how the build container is actually executed by Pipelines isn't documented anywhere. This would be really good information in the documentation to see how to build and design my image structure to work with pipelines.

Also, GitLab has a good documentation with a Keyword reference for the .yml file to assist users to find available bits and pieces in one place. I think if Pipelines would have something that goes that deep into details, that would be great.