Peg yarn version to ensure deterministic builds

"Yarn is fully deterministic as long as all your teammates are using the same Yarn version." (https://classic.yarnpkg.com/blog/2017/05/31/determinism/)

Author: mjgallag

ci/image/Dockerfile

@@ -15,7 +15,7 @@ RUN yum update -y && yum install -y \
 RUN mkdir /usr/share/node && cd /usr/share/node \
   && curl "https://nodejs.org/dist/v12.14.0/node-v12.14.0-linux-$(uname -m | sed 's/86_//; s/aarch/arm/').tar.xz" | tar xJ --strip-components=1 --
 ENV PATH "$PATH:/usr/share/node/bin"
-RUN npm install -g yarn
+RUN npm install -g yarn@1.22.4
 
 RUN curl -L "https://github.com/mvdan/sh/releases/download/v3.0.1/shfmt_v3.0.1_linux_$(uname -m | sed 's/x86_/amd/; s/aarch64/arm/')" > /usr/local/bin/shfmt \
   && chmod +x /usr/local/bin/shfmt