Node Docker Container Cannot Run Yarn

Yarn is part of the official node:8 Docker container. On March 14 (2 days ago), my build script including "yarn build:sb" ran successfully. Today it says:

+ yarn build:sb
bash: yarn: command not found

I did not change the bitbucket-pipelines.yml file. (Actually I did, but I reverted it back). Unfortunately I can't release anything because of this.