Fix Dockerfile paths for go.mod and go.sum

backend/Dockerfile

@@ -4,15 +4,16 @@ WORKDIR /app
 
 RUN apk add --no-cache git nodejs npm
 
-COPY go.mod go.sum ./
+COPY backend/go.mod backend/go.sum ./
 RUN go mod download
 
-COPY . .
+COPY backend ./backend
+COPY frontend ./frontend
 
 RUN npm ci --prefix frontend
 RUN npm run build --prefix frontend
 
-RUN CGO_ENABLED=0 GOOS=linux go build -o /app/server ./cmd/server
+RUN CGO_ENABLED=0 GOOS=linux go build -o /app/server ./backend/cmd/server
 
 FROM alpine:latest
 
@@ -22,7 +23,7 @@ RUN apk add --no-cache ca-certificates tzdata
 
 COPY --from=builder /app/server .
 COPY --from=builder /app/frontend/dist ./static
-COPY --from=builder /app/uploads ./uploads
+COPY --from=builder /app/backend/uploads ./uploads
 
 EXPOSE 8080