cmd/containerboot: PID1 for running tailscaled in a container.

This implements the same functionality as the former run.sh, but in Go
and with a little better awareness of tailscaled's lifecycle.

Also adds TS_AUTH_ONCE, which fixes the unfortunate behavior run.sh had
where it would unconditionally try to reauth every time if you gave it
an authkey, rather than try to use it only if auth is actually needed.
This makes it a bit nicer to deploy these containers in automation, since
you don't have to run the container once, then go and edit its definition
to remove authkeys.

Signed-off-by: David Anderson <danderson@tailscale.com>

Dockerfile

@@ -66,10 +66,12 @@ RUN GOARCH=$TARGETARCH go install -ldflags="\
       -X tailscale.com/version.Long=$VERSION_LONG \
       -X tailscale.com/version.Short=$VERSION_SHORT \
       -X tailscale.com/version.GitCommit=$VERSION_GIT_HASH" \
-      -v ./cmd/tailscale ./cmd/tailscaled
+      -v ./cmd/tailscale ./cmd/tailscaled ./cmd/containerboot
 
 FROM alpine:3.16
 RUN apk add --no-cache ca-certificates iptables iproute2 ip6tables
 
 COPY --from=build-env /go/bin/* /usr/local/bin/
-COPY --from=build-env /go/src/tailscale/docs/k8s/run.sh /usr/local/bin/
+# For compat with the previous run.sh, although ideally you should be
+# using build_docker.sh which sets an entrypoint for the image.
+RUN ln -s /usr/local/bin/containerboot /tailscale/run.sh