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

@@ -1,11 +1,12 @@
 FROM ubuntu:latest
 
 ARG TAILSCALE_VERSION
+ARG TAILSCALE_CHANNEL
 
 RUN apt-get update \
     && apt-get install -y gnupg curl \
-    && curl -fsSL https://pkgs.tailscale.com/stable/ubuntu/focal.gpg | apt-key add - \
-    && curl -fsSL https://pkgs.tailscale.com/stable/ubuntu/focal.list | tee /etc/apt/sources.list.d/tailscale.list \
+    && curl -fsSL https://pkgs.tailscale.com/${TAILSCALE_CHANNEL}/ubuntu/focal.gpg | apt-key add - \
+    && curl -fsSL https://pkgs.tailscale.com/${TAILSCALE_CHANNEL}/ubuntu/focal.list | tee /etc/apt/sources.list.d/tailscale.list \
     && apt-get update \
     && apt-get install -y ca-certificates tailscale=${TAILSCALE_VERSION} dnsutils \
     && rm -rf /var/lib/apt/lists/*