Add explicit "golang:alpine" variant description

golang/variant-alpine.md

@@ -0,0 +1,9 @@
+## `%%IMAGE%%:<version>-alpine`
+
+This image is based on the popular [Alpine Linux project](https://alpinelinux.org), available in [the `alpine` official image](https://hub.docker.com/_/alpine). Alpine Linux is much smaller than most distribution base images (~5MB), and thus leads to much slimmer images in general.
+
+This variant is highly experimental, and *not* officially supported by the Go project (see [golang/go#19938](https://github.com/golang/go/issues/19938) for details).
+
+The main caveat to note is that it does use [musl libc](https://musl.libc.org) instead of [glibc and friends](https://www.etalabs.net/compare_libcs.html), which can lead to unexpected behavior. See [this Hacker News comment thread](https://news.ycombinator.com/item?id=10782897) for more discussion of the issues that might arise and some pro/con comparisons of using Alpine-based images.
+
+To minimize image size, additional related tools (such as `git`, `gcc`, or `bash`) are not included in Alpine-based images. Using this image as a base, add the things you need in your own Dockerfile (see the [`alpine` image description](https://hub.docker.com/_/alpine/) for examples of how to install packages if you are unfamiliar). See also [docker-library/golang#250 (comment)](https://github.com/docker-library/golang/issues/250#issuecomment-451201761) for a longer explanation.