Use numeric UID in Alpine Docker image

Same as PR #2332

packaging/docker/alpine/Dockerfile

@@ -22,5 +22,5 @@ EXPOSE 8080
 ENV LISTEN_ADDR 0.0.0.0:8080
 RUN apk --no-cache add ca-certificates tzdata
 COPY --from=build /go/src/app/miniflux /usr/bin/miniflux
-USER nobody
+USER 65534
 CMD ["/usr/bin/miniflux"]