fix: CI에서 tolchain 의존성 포함하여 Docker 빌드

Dockerfile을 상위 빌드 컨텍스트 방식으로 변경하고,
CI에서 tolchain도 함께 clone하여 빌드 실패 해결.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -1,9 +1,10 @@
 # Stage 1: Build
 FROM golang:alpine AS builder
-WORKDIR /app
-COPY go.mod go.sum ./
+WORKDIR /build
+COPY tolchain/ ./tolchain/
+COPY a301_server/ ./a301_server/
+WORKDIR /build/a301_server
 RUN go mod download
-COPY . .
 RUN CGO_ENABLED=0 GOOS=linux go build -o server .
 
 # Stage 2: Run
@@ -11,6 +12,6 @@ FROM alpine:latest
 RUN apk --no-cache add tzdata ca-certificates
 RUN mkdir -p /data/game
 WORKDIR /app
-COPY --from=builder /app/server .
+COPY --from=builder /build/a301_server/server .
 EXPOSE 8080
 CMD ["./server"]