I made this CLI installer for Unity that will install any semantic version range for OS X (hard coding URL not necessary): https://github.com/zettaforge/unity-installer

I would accept pull requests for Linux and Windows package support.

You can also avoid hard coding any install paths using unity-path: https://github.com/zettaforge/unity-path

I would accept PRs there too to professionalise documentation (emotional appeal was an experiment that brutally failed) or help with marketing.

Both these tools could be useful for customisation of this Docker image. They are thoroughly tested with full coverage :)