Update README to point to correct image

[nvllsvm]

The image should be alpine instead of gliderlabs/alpine.

[andyshinn]

I'm not sure about this. This project is the gliderlabs/alpine image. We supply the tarballs for the official repository. But we were first gliderlabs/alpine and still are. The argument against this is because the specific official docs are at https://github.com/docker-library/docs/tree/master/alpine so those docs would reference alpine while ours would gliderlabs/alpine.

Though, I'll leave this open for more feedback. @tianon thoughts?

[nvllsvm]

My confusion arose when I saw the latest tag in gliderlabs/alpine is 3.4 while the official alpine tag is the latest 3.6 - just like this repo. Will the gliderlabs/alpine image be kept up-to-date with the latest tags?

Additional confusion was felt after seeing that Glider Labs is mentioned as the maintainer in the official image.

[andyshinn]

That is a good question. I think the last two releases were pull requests and maybe they just neglected to update both. I'll get them updated to they are in sync.