Replace npm with yarn as per MBS v-2018-04-23

- Update yarnpkg apt repository only.

- Remove cmdtest if already installed as it conflicts with yarn,
  see yarnpkg/yarn#2821

  (Since Ubuntu 17.04, cmdtest is installed by default.)

musicbrainz-dockerfile/Dockerfile

@@ -23,9 +23,16 @@ RUN git clone --recursive https://github.com/metabrainz/musicbrainz-server.git m
     cd musicbrainz-server && \
     git checkout v-2018-06-30
 
-RUN cd tmp && \
+RUN cd musicbrainz-server && \
+    cp docker/yarn_pubkey.txt /tmp && \
+    cd /tmp && \
+    apt-key add yarn_pubkey.txt && \
+    echo "deb https://dl.yarnpkg.com/debian/ stable main" | tee /etc/apt/sources.list.d/yarn.list && \
+    apt-get update -o Dir::Etc::sourcelist="sources.list.d/yarn.list" -o Dir::Etc::sourceparts="-" -o APT::Get::List-Cleanup="0" && \
     curl -sLO https://deb.nodesource.com/node_8.x/pool/main/n/nodejs/nodejs_8.11.3-1nodesource1_amd64.deb && \
-    dpkg -i nodejs_8.11.3-1nodesource1_amd64.deb
+    dpkg -i nodejs_8.11.3-1nodesource1_amd64.deb && \
+    apt remove -y cmdtest && \
+    apt-get install -y yarn
 
 RUN cd /musicbrainz-server/ && eval $( perl -Mlocal::lib) && cpanm --installdeps --notest .
 RUN eval $( perl -Mlocal::lib) && cpanm --notest Plack::Middleware::Debug::Base \
@@ -52,7 +59,7 @@ ADD scripts/createdb.sh /createdb.sh
 ADD scripts/recreatedb.sh /recreatedb.sh
 ADD scripts/set-token.sh /set-token.sh
 
-RUN cd /musicbrainz-server/ && npm install --only=production \
+RUN cd /musicbrainz-server/ && yarn install --production \
     && eval $( perl -Mlocal::lib) && /musicbrainz-server/script/compile_resources.sh
 
 RUN apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*