How to build for iOS

The unity3d docker project already creates docker images able to build for iOS, but they are not tested yet:

https://gitlab.com/gableroux/unity3d/blob/7ec5008c/.gitlab-ci.yml#L3560

That part of the gitlab-ci.yml built and tagged gableroux/unity3d:2018.2.2f1-ios image so we can use this image to build for iOS.

I did not try it yet, but I know for sure that building for iOS is a special case as Unity generates an xcode project with a xcodeproj file. Once this is generated through artifacts, we can use a different gitlab-ci job to reuse the artifact from previous job and build the xcode project using fastlane running on a personal MacOS runner.

We should provide an example in this project to do so, or at least document how we can do this.