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/Dockerfile.qpkg

@@ -0,0 +1,9 @@
+FROM ubuntu:20.04
+
+RUN apt-get update -y && \
+    apt-get install -y --no-install-recommends \
+    git-core \
+    ca-certificates
+RUN git clone https://github.com/qnap-dev/QDK.git
+RUN cd /QDK && ./InstallToUbuntu.sh install
+ENV PATH="/usr/share/QDK/bin:${PATH}"