fix: remove redundant apk gcc install, golang:1.20-alpine already has gcc

Dockerfile

@@ -10,9 +10,8 @@ ENV GOPROXY=https://goproxy.cn,direct
 ENV GOSUMDB=off
 
 
-# Build binary (CGO required for go-fitz/MuPDF)
-RUN apk add --no-cache gcc musl-dev && \
-    go mod download && \
+# Build binary (CGO required for go-fitz/MuPDF bundled static libs)
+RUN go mod download && \
     CGO_ENABLED=1 GOOS=linux go build -ldflags="-s -w" -o doc_ai ./main.go
 
 # Runtime stage