Update Helm chart and Dockerfile

Signed-off-by: James Munnelly <james@munnelly.eu>

Dockerfile

@@ -1,7 +1,25 @@
-FROM golang:1.12.1
+FROM golang:1.12.4-alpine AS build_deps
 
-COPY . /workspace
+RUN apk add --no-cache git
 
 WORKDIR /workspace
-RUN go build
+ENV GO111MODULE=on
 
+COPY go.mod .
+COPY go.sum .
+
+RUN go mod download
+
+FROM build_deps AS build
+
+COPY . .
+
+RUN CGO_ENABLED=0 go build -o webhook -ldflags '-w -extldflags "-static"' .
+
+FROM alpine:3.9
+
+RUN apk add --no-cache ca-certificates
+
+COPY --from=build /workspace/webhook /usr/local/bin/webhook
+
+ENTRYPOINT ["webhook"]