improve Dockerfile: new golang and cache dependencies in extra layer

Dockerfile

@@ -1,15 +1,20 @@
-FROM golang:1.17-alpine AS builder
+FROM golang:1.19.4-alpine AS builder
 
 WORKDIR /build
-COPY . /build/
-
 ENV CGO_ENABLED=0
 
+# cache dependencies
+# from https://github.com/montanaflynn/golang-docker-cache
+COPY go.mod go.sum ./
+RUN go mod graph | awk '{if ($1 !~ "@") print $2}' | xargs -r go get
+
+COPY . ./
+
 RUN go build -v .
 
 FROM busybox
 
-COPY --from=builder /build/fritzbox_exporter /usr/local/bin/fritzbox_exporter
+COPY --from=builder /build/fritzbox_exporter /app/fritzbox_exporter
 
 EXPOSE 9133
-ENTRYPOINT /usr/local/bin/fritzbox_exporter
+ENTRYPOINT /app/fritzbox_exporter