Add dockerfile to build inside a docker container

[NicolasMahe]

This PR adds a dockerfile and instructions to use docker to run and build this library inside a controlled environment.

I didn't want to install GopherJS on my machine because of the old go version required, so I end up running it in a container.

Quick guide:

# build the image
docker build .

# run `yarn publish`
docker run --rm -it -v $(PWD)/dist:/project/dist -v $(PWD)/types:/project/types $(docker build -q .) yarn publish

[jordaaash]

Thanks for the PR! This is a good idea, I'll test it out and see if it works for me.

[NicolasMahe]

i changed this because it was not working inside the debian docker container.