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/build-qpkg.sh

@@ -0,0 +1,19 @@
+#!/bin/bash
+
+set -eu
+
+# Clean up folders and files created during build.
+function cleanup() {
+    rm -rf /Tailscale/$ARCH
+    rm -f /Tailscale/sed*
+    rm -f /Tailscale/qpkg.cfg
+}
+trap cleanup EXIT
+
+mkdir -p /Tailscale/$ARCH
+cp /tailscaled /Tailscale/$ARCH/tailscaled
+cp /tailscale /Tailscale/$ARCH/tailscale
+
+sed "s/\$QPKG_VER/$TSTAG-$QNAPTAG/g" /Tailscale/qpkg.cfg.in > /Tailscale/qpkg.cfg
+
+qbuild --root /Tailscale --build-arch $ARCH --build-dir /out