add initial alpine packaging

notes:
- to build the package, you must use the cargo version from the edge branch
  (by building on edge or by installing it manually)
- building from git requires some work (abuild supports snapshots for getting
  the release from git, but the version number would remain unchanged)
- the apkbuild doesn't include any packaging tests (as i don't know what to include)

alpine/continuwuity.pre-install

@@ -0,0 +1,4 @@
+#!/bin/sh
+addgroup -S continuwuity 2>/dev/null
+adduser -S -D -H -h /var/lib/continuwuity -s /sbin/nologin -G continuwuity -g continuwuity continuwuity 2>/dev/null
+exit 0