Download npm and yarn from Alpine packages repository (#4242)

Dockerfile

@@ -12,6 +12,7 @@ EXPOSE 3000 4000
 WORKDIR /mastodon
 
 RUN echo "@edge https://nl.alpinelinux.org/alpine/edge/main" >> /etc/apk/repositories \
+ && echo "@edge https://nl.alpinelinux.org/alpine/edge/community" >> /etc/apk/repositories \
  && apk -U upgrade \
  && apk add -t build-dependencies \
     build-base \
@@ -38,7 +39,7 @@ RUN echo "@edge https://nl.alpinelinux.org/alpine/edge/main" >> /etc/apk/reposit
     protobuf \
     su-exec \
     tini \
- && npm install -g npm@3 && npm install -g yarn \
+    yarn@edge \
  && update-ca-certificates \
  && rm -rf /tmp/* /var/cache/apk/*