Use the official Alpine Linux base image
Use the official Alpine Linux base image

This works around a build failure with the gliderlabs/alpine image [1]
and matches the Docker image source we use for h. This change is also
recommended by Glider Labs [2] who maintain both images.

The resulting image is slightly larger (167MB vs 160MB for latest
released image) but there is plenty of room to reduce the image size in other
ways.

[1] https://github.com/gliderlabs/docker-alpine/issues/464
[2] https://github.com/gliderlabs/docker-alpine/blob/71099f56701722443807578f1771b07670399c46/docs/build.md#differences

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