Add Dockerfile for usage with Docker

[mickdekkers]

By adding a Dockerfile to the project, users can easily run the application without needing to set up a local environment or install dependencies. This makes it faster and easier to start using it, especially for people not familiar with Python development.

Partially solves https://github.com/github-tooling/ghtopdep/issues/14: users can now build a Docker image locally and use it that way, but it might still be nice to publish it to Docker Hub too at some point (which would let people use the app without needing to clone the repo first).

[korthout]

Great idea @mickdekkers 👍

I gave this a try because I was running into a bug (#30) when running ghtopdep directly on my machine.

However, following these steps resulted in an error:

gh repo clone https://github.com/github-tooling/ghtopdep
cd ghtopdep
gh pr checkout 29
docker build . -t ghtopdep
docker run --rm -it ghtopdep --help

Resulted in error:

python: can't open file '/app/main.py': [Errno 2] No such file or directory