Support other/edge Alpine versions

[cbrunnkvist]

Newly generated images are always (implicitly) based on :latest - the FROM base image specifies just alpine. It would be complementary (well, quite neat) if we could use the tag specified on cmd.cat/[...]:${tag} to change the base.

Perhaps not necessarily fully parameterize it, but at least add support for :edge to use FROM alpine:edge as base. 🌻