Added build scripts (Ubuntu, Mac, Linux) and Dockerfile (Ubuntu 18.04)

[AlbertoCasasOrtiz]

Fixes issue #3397 and Fixes issue #3369

Brief summary of changes

Copied build scripts here, and use this folder as a permanent place (currently the scripts are in my personal account as gists, so we discussed in #3397 that this could be a better place).

Testing I've completed

build-scripts: Tested in previous iterations of the scripts, now I am only copying them here.
docker script: Tested by downloading generated artifact and executing it in an Ubuntu 18.04 VM.

Looking for feedback on...

Should we add opensim-gui build scripts here too, or should we create another script folder in opensim-gui's repository?

The docker script is for opensim-core. Should I create another for opensim-gui?

CHANGELOG.md (choose one)

• no need to update because internal scripts.

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This change is 

[halleysfifthinc]

There's a typo in the folder name (buiild scripts), and I would recommend not using spaces, which are a pain to deal with on the command line. The main build scripts could just live directly in the scripts folder.

[AlbertoCasasOrtiz]

Thank you @halleysfifthinc for reviewing this. I agree, there should not be spaces in the path, and good catch the typo. I have renamed the folder buiild scripts to build. Same with docker/Ubuntu 18.04, it is now docker/ubuntu18.04. I think we should have the build scripts separated, just in case in the future more scripts are added to this folder, we have them organized and they are easier to find.