Switch to alpine for the docker image

[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).

[AGhost-7]

I think that size is less important in this case. If you're going to use this for testing, you're probably going to need to use it for debugging, and using something you're familiar with is a better choice. I think a lot of people are more familiar with Debian.

[ian-kent]

Thanks @lmakarov, and thanks for #109

Does the version provided by Rockerfile help? It's only 15 MB, and is the 'official' one hosted at Docker Hub

I think I'd be inclined to agree with @AGhost-7, but I won't be affected by it! Dockerfile is intended as a development resource, while the Rockerfile version provides something aimed more at deployment.

[ian-kent]

I'll close this, I think it's been superseded by #118