Copy upstream response into http writer

As an optimization, the response body for the HTTP runner can
stream data back, instead of buffering it all in memory and
then sending it in one shot.

This change means replacing an io.ReadAll with an io.Copy, as a
result, it's possible that some callers will no longer receive
a content-length, but a stream of data with a length of -1.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

Dockerfile

@@ -1,8 +1,8 @@
-FROM scratch as cache
+FROM scratch AS cache
 
 COPY bin    .
 
-FROM scratch as ship
+FROM scratch AS ship
 
 ARG TARGETPLATFORM
 ARG BUILDPLATFORM
@@ -11,4 +11,11 @@ ARG TARGETARCH
 
 COPY --from=cache /fwatchdog-$TARGETARCH ./fwatchdog
 
+LABEL org.label-schema.license="MIT" \
+      org.label-schema.vcs-url="https://github.com/openfaas/of-watchdog" \
+      org.label-schema.vcs-type="Git" \
+      org.label-schema.name="openfaas/of-watchdog" \
+      org.label-schema.vendor="openfaas" \
+      org.label-schema.docker.schema-version="1.0"
+
 ENTRYPOINT ["/fwatchdog"]

README.md

@@ -78,13 +78,13 @@ Example usage for testing:
 * Forward to an Nginx container:
 
 ```
-$ go build ; mode=http port=8081 fprocess="docker run -p 80:80 --name nginx -t nginx" upstream_url=http://127.0.0.1:80 ./of-watchdog
+$ go build && mode=http port=8081 fprocess="docker run -p 80:80 --name nginx -t nginx" upstream_url=http://127.0.0.1:80 ./of-watchdog
 ```
 
 * Forward to a Node.js / Express.js hello-world app:
 
 ```
-$ go build ; mode=http port=8081 fprocess="node expressjs-hello-world.js" upstream_url=http://127.0.0.1:3000 ./of-watchdog
+$ go build && mode=http port=8081 fprocess="node expressjs-hello-world.js" upstream_url=http://127.0.0.1:3000 ./of-watchdog
 ```
 
 Cons:

executor/http_runner.go

@@ -183,11 +183,9 @@ func (f *HTTPFunctionRunner) Run(req FunctionRequest, contentLength int64, r *ht
 		if res.Body != nil {
 			defer res.Body.Close()
 
-			bodyBytes, bodyErr := io.ReadAll(res.Body)
-			if bodyErr != nil {
-				log.Println("read body err", bodyErr)
+			if _, err := io.Copy(w, res.Body); err != nil {
+				log.Printf("Error copying response body: %s", err)
 			}
-			w.Write(bodyBytes)
 		}
 
 		// Exclude logging for health check probes from the kubelet which can spam