Use golang:1.7-alpine base image. Resolves #108

[lmakarov]

Alpine based image is more than two times smaller than the standard (Debian based) one.
The resulting mailhog image size is 280.1 MB (using golang:1.7-alpine) vs 691.6 MB (using golang:1.7).

[mwgamble]

Instead of using the golang image as a base, and go geting mailhog in the dockerfile, why not just create a basic dockerfile based on the official alpine image, and use wget to download the binary from github?

[blueimp]

I propose the Dockerfile in my pull request here, which optimises image size to just 18.93 MB, is compatible with Docker Hub Automated Builds and also fixes the "release message" feature.

[ian-kent]

I'll close this since I've merged the one from @blueimp, but thanks all the same @lmakarov 😄