Add SeaweedFS Alpine package build

packaging/alpine/local/seaweedfs/seaweedfs.pre-install

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+addgroup -S seaweedfs 2>/dev/null
+adduser -S -D -H -h /var/lib/seaweedfs -s /sbin/nologin \
+	-G seaweedfs -g seaweedfs seaweedfs 2>/dev/null
+
+exit 0