Make GOPROXY a build arg (proxy.golang.org is blocked on some networks)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Dockerfile

@@ -12,6 +12,11 @@ ARG VERSION=dev
 RUN TEFTER_VERSION=$VERSION npx vite build
 
 FROM docker.io/library/golang:1.25-alpine AS build
+# Overridable for hosts where proxy.golang.org (Google) is unreachable —
+# set e.g. GOPROXY=https://goproxy.io,direct in .env.docker. Module
+# integrity is still verified against the committed go.sum either way.
+ARG GOPROXY=https://proxy.golang.org,direct
+ENV GOPROXY=$GOPROXY
 WORKDIR /src/server
 COPY server/go.mod server/go.sum ./
 RUN go mod download