add dockerfile

Dockerfile

@@ -0,0 +1,13 @@
+FROM golang:rc-alpine3.13 AS builder
+RUN go get github.com/sberk42/fritzbox_exporter/
+
+FROM alpine:latest
+ARG USERNAME PASSWORD GWURL
+ENV USERNAME ${USERNAME} && \
+PASSWORD ${PASSWORD} && \
+GWURL ${GWURL}
+WORKDIR /root/
+COPY --from=builder /go/bin/fritzbox_exporter . 
+COPY metrics.json metrics-lua.json ./
+EXPOSE 9042
+ENTRYPOINT ./fritzbox_exporter -gateway-url ${GWURL} -password ${PASSWORD} -username ${USERNAME} -listen-address 0.0.0.0:9042