docker高山图像不发现php7.1-xsl

我正在尝试使用包含php7-xsl的 Alpine Linux创build一个容器。

我的Dockerfile如下所示：

 FROM composer RUN echo "@edge http://liskamm.alpinelinux.uk/edge/main" >> /etc/apk/repositories; \ echo "@testing http://dl-4.alpinelinux.org/alpine/edge/testing" >> /etc/apk/repositories # Install xls extension RUN apk --no-cache --update add libxslt-dev php7.1-xsl 

当我尝试构build容器时，出现以下错误：

 ERROR: unsatisfiable constraints: php7.1-xsl (virtual): provided by: php7-xsl required by: world[php7.1-xsl] The command '/bin/sh -c apk --no-cache --update add libxslt-dev php7.1-xsl@testing' returned a non-zero code: 1 

我怎样才能解决这个问题？