Why wrapping a web project and making it deploy able to any server seems so hard with Docker?

I've been working on an open source project of mine that recently got featured on Laravel News https://laravel-news.com/unified-transform-open-source-school-management-platform .

The common problem I faced in Issues is to make this web app seamlessly deploy able to any platform due to different PHP and composer versions of users. So I wanted to implement a Container solution like Docker. But Docker seems pretty hard to me to work with.

It'll be great if anybody can suggest me a simple working steps to work with Docker. Thanks.