release/dist/qnap: add qnap target builder

Creates new QNAP builder target, which builds go binaries then uses
docker to build into QNAP packages. Much of the docker/script code
here is pulled over from https://github.com/tailscale/tailscale-qpkg,
with adaptation into our builder structures.

The qnap/Tailscale folder contains static resources needed to build
Tailscale qpkg packages, and is an exact copy of the existing folder
in the tailscale-qpkg repo.

Builds can be run with:
```
sudo ./tool/go run ./cmd/dist build qnap
```

Updates tailscale/tailscale-qpkg#135

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

release/dist/qnap/Tailscale/shared/ui/.htaccess

@@ -0,0 +1,2 @@
+Options +ExecCGI
+AddHandler cgi-script .cgi

release/dist/qnap/Tailscale/shared/ui/index.cgi

@@ -0,0 +1,5 @@
+#!/bin/sh
+CONF=/etc/config/qpkg.conf
+QPKG_NAME="Tailscale"
+QPKG_ROOT=$(/sbin/getcfg ${QPKG_NAME} Install_Path -f ${CONF} -d"")
+exec "${QPKG_ROOT}/tailscale" --socket=/tmp/tailscale/tailscaled.sock web --cgi --prefix="/cgi-bin/qpkg/Tailscale/index.cgi/"