fix: docker cache permission issue (#57)

Dockerfile

@@ -11,8 +11,17 @@ RUN CGO_ENABLED=0 GOOS=linux go build -o saveany-bot .
 
 FROM alpine:latest
 
+RUN addgroup -S saveany && adduser -S saveany -G saveany
+
 WORKDIR /app
 
+RUN mkdir -p /app/data /app/downloads /app/cache && \
+    chown -R saveany:saveany /app
+
 COPY --from=builder /app/saveany-bot .
 
-CMD ["./saveany-bot"]
+RUN chmod +x /app/saveany-bot
+
+USER saveany
+
+CMD ["./saveany-bot"]