Add Tailscale unstable channel and repo HEAD to integration tests

In preparation for the implementation of the new TS2021 protocol (Tailscale control protocol v2) we are expanding the test infrastructure

Dockerfile.tailscale-HEAD

@@ -0,0 +1,21 @@
+FROM golang:latest
+
+RUN apt-get update \
+    && apt-get install -y ca-certificates dnsutils git \
+    && rm -rf /var/lib/apt/lists/*
+
+
+RUN git clone https://github.com/tailscale/tailscale.git
+
+WORKDIR tailscale
+
+RUN sh build_dist.sh tailscale.com/cmd/tailscale
+RUN sh build_dist.sh tailscale.com/cmd/tailscaled
+
+RUN cp tailscale /usr/local/
+RUN cp tailscaled /usr/local/
+
+ADD integration_test/etc_embedded_derp/tls/server.crt /usr/local/share/ca-certificates/
+RUN chmod 644 /usr/local/share/ca-certificates/server.crt 
+
+RUN update-ca-certificates