Add alpine image

[nikeee]

FROM nginx seems to pull a debian-based image.

It would be nice to have something like socialengine/nginx-spa:alpine that uses nginx:stable-alpine.

nginx docs: https://hub.docker.com/_/nginx

[nikeee]

I implemented this in a fork:
https://github.com/nikeee/docker-nginx-spa

It also supports ARM (e.g. for raspberry pi).