revert: backend Dockerfile just copies the locally-built server binary, no in-container Go build

backend/Dockerfile

@@ -1,16 +1,3 @@
-# ---- مرحله‌ی build ----
-FROM golang:1.25-alpine AS builder
-WORKDIR /app
-
-RUN sed -i 's|dl-cdn.alpinelinux.org|mirror.mobinhost.com|g' /etc/apk/repositories
-
-COPY go.mod go.sum ./
-RUN go mod download
-
-COPY . .
-RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o /out/server ./cmd/api
-
-# ---- مرحله‌ی اجرا (ایمیج نهایی سبک) ----
 FROM alpine:3.20
 
 RUN sed -i 's|dl-cdn.alpinelinux.org|mirror.mobinhost.com|g' /etc/apk/repositories \
@@ -18,7 +5,7 @@ RUN sed -i 's|dl-cdn.alpinelinux.org|mirror.mobinhost.com|g' /etc/apk/repositori
 
 WORKDIR /app
 
-COPY --from=builder /out/server ./server
+COPY server ./server
 RUN chmod +x /app/server
 
 RUN mkdir -p /app/uploads/photos