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>
