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.confd

@@ -0,0 +1,3 @@
+supervisor=supervise-daemon
+export CONTINUWUITY_CONFIG=/etc/continuwuity/continuwuity.toml
+