Fix Dockerfile by re-adding GOPROXY_URL argument for clarity

Dockerfile

@@ -3,6 +3,7 @@ ARG GOPROXY_URL=https://gitea.nehmer.net/api/packages/torben/go,https://proxy.go
 
 FROM golang:${GO_VERSION}-bookworm AS build
 WORKDIR /src
+ARG GOPROXY_URL
 ENV GOPROXY=${GOPROXY_URL}
 
 COPY go.mod ./