Reworked shell hello world example

* Reworked example to be a "true" shell example that is considerable different than the helloworld-go example
* Added examples for kn usage

(Fixed according to review comments)

docs/serving/samples/hello-world/helloworld-shell/Dockerfile

@@ -1,32 +1,13 @@
-# Use the official Golang image to create a build artifact.
-# This is based on Debian and sets the GOPATH to /go.
-# https://hub.docker.com/_/golang
-FROM golang:1.13 as builder
-
-# Create and change to the app directory.
-WORKDIR /app
-
-# Retrieve application dependencies using go modules.
-# Allows container builds to reuse downloaded dependencies.
-COPY go.* ./
-RUN go mod download
-
-# Copy local code to the container image.
-COPY invoke.go ./
-
-# Build the binary.
-# -mod=readonly ensures immutable go.mod and go.sum in container builds.
-RUN CGO_ENABLED=0 GOOS=linux go build -mod=readonly -v -o server
-
 # Use the official Alpine image for a lean production container.
 # https://hub.docker.com/_/alpine
-# https://docs.docker.com/develop/develop-images/multistage-build/#use-multi-stage-builds
 FROM alpine:3
-RUN apk add --no-cache ca-certificates
 
-# Copy the binary to the production image from the builder stage.
-COPY --from=builder /app/server /server
-COPY script.sh ./
+# Update & install netcat (nc)
+RUN apk update \
+ && apk add netcat-openbsd
+
+# Copy over the service script
+COPY script.sh /
 
-# Run the web service on container startup.
-CMD ["/server"]
+# Start up the webserver
+CMD [ "/bin/sh", "/script.sh" ]