Create .dockerignore

This PR adds a [.dockerignore file](https://docs.docker.com/engine/reference/builder/#dockerignore-file). What it does is specify which directories don't have to copied into the Docker context when building. Essentially, this is both a security feature (don't inadvertently include secure files into the Docker image), as well as an optimization (less files in the Docker context means faster builds).

As far as I could tell, the `test` folder is not used anywhere in the Docker image, but if I'm mistaken feel free to ignore this PR.

Author: ErikSchierboom

.dockerignore

@@ -0,0 +1 @@
+test