Resolve "How to install Unity3d in docker using 'Linux Download Assistant'" building 2017 and 2018 versions

Strongly based on !3 (closed) MattDahEpic's work, moved dockerfile changes to its own file

Building unity docker images with All components in a single image on gitlab-ci using shared runners seem to hit hard limits, these images fail with one of these error:

https://gitlab.com/gableroux/unity3d/pipelines/25235317/failures

Job's log exceeded limit of 4194304 bytes.

no space left on device:

Error processing tar file(exit status 1): write /opt/Unity/Editor/Data/PlaybackEngines/WebGLSupport/BuildTools/lib/UnityNativeJs/UnityNative.js: no space left on device
Error processing tar file(exit status 1): write /opt/Unity/Editor/Data/PlaybackEngines/WebGLSupport/BuildTools/Emscripten_FastComp_Linux/clang-5.0: no space left on device
Error processing tar file(exit status 1): write /opt/Unity/Editor/Data/PlaybackEngines/WindowsStandaloneSupport/Variations/win32_development_mono/UnityPlayer_Win32_development_x86.pdb: no space left on device

Note that building manually All components will work, but will lead to a ~16gb docker image which is kind of insane ;)

this merge request builds all available versions from 2017 to 2018 with most of the Components combination.

the .gitlab-ci.yml file is getting huge, I will fix this in a new issue.

Closes #8 (closed)