Use Alpine Linux Base Docker Image

[westy92]

Currently the Dockerfile is using a base image of golang:1.9 (274 MB).
This could be switched to an Alpine Linux version golang:1.9-alpine (83 MB) to drastically reduce the image size.

https://hub.docker.com/r/library/golang/tags/

[fiorix]

I'm fine with that. PR?

[fiorix]

See #268

[justgage]

Made a PR on the new repo: apilayer/freegeoip#1