Fix Docker build error: explicitly set WORKDIR in builder stage and fix binary path

Dockerfile

@@ -4,13 +4,15 @@ FROM golang:1.22-alpine AS builder
 # SQLite 및 CGO 빌드에 필요한 패키지 설치
 RUN apk add --no-cache gcc musl-dev
 
-# Cache go modules
-COPY backend/go.mod backend/go.sum ./backend/
-RUN cd backend && go mod download
+WORKDIR /build
 
-COPY backend/ ./backend/
-# We'll build the go binary inside the backend dir and put it in /app/main. Remove debug info for smaller binary.
-RUN cd backend && CGO_ENABLED=1 go build -ldflags="-s -w" -o /app/main main.go
+# Cache go modules
+COPY backend/go.mod backend/go.sum ./
+RUN go mod download
+
+COPY backend/ ./
+# CGO_ENABLED=1 is required for sqlite3
+RUN CGO_ENABLED=1 go build -ldflags="-s -w" -o main main.go
 
 # 2. Final Minimal Image (Python + Go binary + Frontend)
 FROM python:3.10-slim
@@ -31,7 +33,7 @@ COPY worker/requirements.txt ./worker/
 RUN pip install --no-cache-dir -r worker/requirements.txt
 
 # Copy Go binary from builder
-COPY --from=builder /app/main ./main
+COPY --from=builder /build/main ./main
 
 # Copy worker script
 COPY worker/ ./worker/