fix : specify the default hostname if docker builds with a proxy

Dockerfile

@@ -41,8 +41,8 @@ COPY --from=builder /app/.next/server ./.next/server
 EXPOSE 3000
 
 CMD if [ -n "$PROXY_URL" ]; then \
-        if [ -z "$HOSTNAME" ]; then
-          HOSTNAME="127.0.0.1"
+        if [ -z "$HOSTNAME" ]; then \
+          export HOSTNAME="127.0.0.1" \
         fi; \
         protocol=$(echo $PROXY_URL | cut -d: -f1); \
         host=$(echo $PROXY_URL | cut -d/ -f3 | cut -d: -f1); \