No alpine image for arm32v7

[Serkan-devel]

An arm port for alpine linux is missing on https://hub.docker.com/u/arm32v7/

[tianon]

It's in arm32v6 instead (since Alpine chose to support minimum v6 instead of v7, especially for the raspberry pis). Also, those Alpine multiarch images are not officially supported fully yet, see gliderlabs/docker-alpine#304.