Allow LISTEN_ADDRESS environment variable

Dockerfile

@@ -2,11 +2,14 @@ FROM golang:rc-alpine3.13 AS builder
 RUN go get github.com/sberk42/fritzbox_exporter/
 
 FROM alpine:latest
-ENV USERNAME="username"
-ENV PASSWORD="password"
-ENV GATEWAY_URL="http://fritz.box:49000"
+ENV USERNAME username
+ENV PASSWORD password
+ENV GATEWAY_URL http://fritz.box:49000
+ENV LISTEN_ADDRESS 0.0.0.0:9042
 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
+ENTRYPOINT [ "./fritzbox_exporter" ]
+CMD ./fritzbox_exporter -username $USERNAME -password $PASSWORD
+ENTRYPOINT ./fritzbox_exporter -gateway-url ${GWURL} -password ${PASSWORD} -username ${USERNAME} -listen-address ${LISTEN_ADDRESS}