Upgrade to Yarn 2.

.gitignore

@@ -15,3 +15,11 @@ public/data/
 public/packs-test/
 public/packs/
 test/reports/
+
+# https://yarnpkg.com/getting-started/qa#which-files-should-be-gitignored
+.yarn/*
+!.yarn/patches
+!.yarn/plugins
+!.yarn/releases
+!.yarn/sdks
+!.yarn/versions

.yarnrc.yml

@@ -0,0 +1,2 @@
+yarnPath: ".yarn/releases/yarn-berry.cjs"
+nodeLinker: node-modules

config/docker/Dockerfile.danbooru

@@ -38,7 +38,7 @@ ARG DATABASE_URL=postgresql://0.0.0.0
 ARG DANBOORU_SECRET_KEY_BASE=1234
 ARG RAILS_ENV=production
 RUN bin/rails assets:precompile && ln -sf packs public/packs-test
-RUN rm -rf node_modules log tmp && mkdir log tmp
+RUN rm -rf node_modules log tmp .yarn/cache && mkdir log tmp
 
 FROM ubuntu:20.10