Support arm64 for watchdog release

Signed-off-by: Alex Ellis <alexellis2@gmail.com>

watchdog/Dockerfile

-FROM golang:1.7.5
+FROM golang:1.8.3
 RUN mkdir -p /go/src/github.com/alexellis/faas/watchdog
 WORKDIR /go/src/github.com/alexellis/faas/watchdog
 
@@ -18,4 +18,5 @@ RUN go test
 # Stripping via -ldflags "-s -w" 
 RUN CGO_ENABLED=0 GOOS=linux go build -a -ldflags "-s -w" -installsuffix cgo -o watchdog . \
     && GOARM=6 GOARCH=arm CGO_ENABLED=0 GOOS=linux go build -a -ldflags "-s -w" -installsuffix cgo -o watchdog-armhf . \
+    && GOARCH=arm64 CGO_ENABLED=0 GOOS=linux go build -a -ldflags "-s -w" -installsuffix cgo -o watchdog-arm64 . \
     && GOOS=windows CGO_ENABLED=0 go build -a -ldflags "-s -w" -installsuffix cgo -o watchdog.exe .

watchdog/build.sh

@@ -14,6 +14,7 @@ docker create --name buildoutput functions/watchdog:build echo
 
 docker cp buildoutput:/go/src/github.com/alexellis/faas/watchdog/watchdog ./fwatchdog
 docker cp buildoutput:/go/src/github.com/alexellis/faas/watchdog/watchdog-armhf ./fwatchdog-armhf
+docker cp buildoutput:/go/src/github.com/alexellis/faas/watchdog/watchdog-arm64 ./fwatchdog-arm64
 docker cp buildoutput:/go/src/github.com/alexellis/faas/watchdog/watchdog.exe ./fwatchdog.exe
 
 docker rm buildoutput