Build docker image using official golang as base

Building from official golang images has the advantage that vulcand is built with latest golang version (aside it is a supported base image).

The image size is only a few Mb bigger:
```
REPOSITORY                                 TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
mailgun/vulcand                            latest              6cda9a942e87        6 hours ago         458.1 MB
golang-vulcan                              latest              c6a166c9c00e        17 minutes ago      482.5 MB
```