Add a Dockerfile directly in the project

[sharkoz]

This allows to build automatically images from the source repo, and to offer pre-built images to users.
After merging, you can add the repo to docker hub for automated building, like I did there :
https://cloud.docker.com/repository/docker/cosme/orion-web
This provides the best experience for users.

[sharkoz]

This should be ready to merge. The image is self contained and can be build on docker hub. Example : https://cloud.docker.com/repository/docker/cosme/orion-web
It includes NGINX so there is no need for a separate nginx container. Nginx directly hosts the static files so no need for a dedicated static node server.

Also to prevent problems with the mapbox TOS, as mentionned in #3 , and as the api key right now has to be included during build, this PR also changes the map tiles for a free to use tiles provider, namely http://maps.stamen.com/

It can be deployed in compose like this :
orion-web:
restart: always
image: cosme/orion-web
depends_on:
- orion-server
ports:
- "80:80"

Or :
docker run --restart always -e LISTEN_PORT=80 -e SERVER_PORT=80 -e SERVER_URL=orion-server cosme/orion-web

[LINKIWI]

I believe this use case is already covered by https://github.com/LINKIWI/orion-docker

[sharkoz]

This is not the same, having the dockerfile alongside the repo allows it to be always up to date, it is a lot cleaner than having a separate repo that users need to clone, and then build the image themselves. With this pr the image can be built directly by Docker hub. This makes things easier for you and your users.