It turns a Dockerfile into an executable script, so that by executing the Dockerfile, the shebang invokes docker to build and run the file.

Pretty neat if you're using Dockerfiles, but also highly non-standard so you wouldn't use it in your company repo (unless you want to increase the "what-the-fuck" level of your repo).

It's more of a "look, this is cool" kind of a thing if you're a Linux and container user.