Dockerfile試圖'apk添加作曲家'失敗？

我試圖學習Docker，並遇到了一個障礙。我的搬運工文件如下Dockerfile試圖'apk添加作曲家'失敗？

# Base image of a nginx box 
FROM richarvey/nginx-php-fpm:latest 

ARG VERSION=0.0.0 
ENV APP=jewel 
ENV WEBROOT=/var/www/html/JEWEL_WEB_ROOT/ 

WORKDIR /var/www/html 
COPY ./ ./ 

# Install composer libs 
RUN echo "ipv6" >> /etc/modules 
#RUN ["apk","update"] 
RUN ["apk","add","composer", "--no-cache"] 
RUN ["composer","install"] 

ENV PORT=80 
EXPOSE 80

當構建過程獲取到APK補充，我得到如下：

Step 8/11 : RUN apk add composer --no-cache 
---> Running in ffa506b1a07f 
fetch http://dl-cdn.alpinelinux.org/alpine/v3.4/main/x86_64/APKINDEX.tar.gz 
fetch http://dl-cdn.alpinelinux.org/alpine/v3.4/community/x86_64/APKINDEX.tar.gz 
fetch http://nl.alpinelinux.org/alpine/edge/testing/x86_64/APKINDEX.tar.gz 
ERROR: unsatisfiable constraints: 
    composer-1.5.2-r0: 
    masked in: @testing 
    satisfies: world[composer] 
ERROR: Service 'web' failed to build: The command 'apk add composer --no-cache' returned a non-zero code: 1

我檢查包裝清單（上pkgs.alpinelinux.org），並且有一個似乎符合標準的軟件包，它似乎在尋找？