Extract Docker integration test variables to separate YAML variable template file

[stijnmoreels]

Is your feature request related to a problem? Please describe.
We use several test variables when running the Docker integration tests. This goes from different HTTP ports that the project should expose, to image names and labels. These values are the same in the CI and release build.

Describe the solution you'd like
Extract the Docker integration test variables from the ci-build.yml and nuget-release.yml YAML builds and create a new YAML variable template file in /build/variables/test-docker.yml. This file can be referenced in both builds.

[stijnmoreels]

This seems to be already extracted into the /variables/test.yml.