Add Dockerfile for WebIDE

[ninoseki]

This PR adds Dockerfile for WebIDE.
Using Docker is one of the most convenient ways to run an app. I believe providing the official Dockerfile helps a lot.

[KOLANICH]

Usage of Docker for everything is just a cargo cult. If Google uses it for its highload services it doesn't mean that you should use it. If it should be used or not depends on a lot of factors. It is more like a prescribtion-use medicine rather than a silver bullet.

[generalmimon]

1. I don't understand why are you installing git into the container and then cloning the repo from inside, when this Dockerfile is part of the cloned repository (meaning that the kaitai_struct_webide repo has already been cloned)?

2. Another problem is that you'll always get the main branch (master) by doing this, which will make it impossible to use this Dockerfile for Web IDE development (like building the image from locally changed files).

   If fetching the remote master branch was your intent, may I ask what's the point of running the Web IDE server with Docker instead of just going to https://ide.kaitai.io/devel/ (or alternatively fetching git clone https://github.com/kaitai-io/ide-kaitai-io.github.io and running a local server in the /devel/ folder).