Upgrade to Yarn v3

Follow the steps at https://yarnpkg.com/getting-started/migration to upgrade to Yarn v3:

 1. `yarn set version berry`
 2. Edit `.yarnrc.yml` to add `nodeLinker: node-modules`.
 3. Run `yarn install` to migrate lockfile
 4. Update `.gitignore` (see comment)
 5. Update Dockerfile (see Via PRs for more details)

This follows changes in the hypothesis/via repo
(hypothesis/via#1153,
hypothesis/via#1155).

.gitignore

@@ -34,3 +34,13 @@ mail/
 tests/py3-actual-failures.txt
 
 .devdata.env
+
+# See https://yarnpkg.com/getting-started/qa#which-files-should-be-gitignored.
+# This is the "If you're not using Zero-Installs" list.
+.pnp.*
+.yarn/*
+!.yarn/patches
+!.yarn/plugins
+!.yarn/releases
+!.yarn/sdks
+!.yarn/versions

.yarnrc.yml

@@ -0,0 +1,2 @@
+nodeLinker: node-modules
+yarnPath: .yarn/releases/yarn-3.6.1.cjs

Dockerfile

@@ -5,8 +5,9 @@ ENV NODE_ENV production
 
 # Install dependencies.
 WORKDIR /tmp/frontend-build
-COPY package.json yarn.lock ./
-RUN yarn install --frozen-lockfile
+COPY package.json .yarnrc.yml yarn.lock ./
+COPY .yarn ./.yarn
+RUN yarn install --immutable
 
 # Build h js/css.
 COPY .babelrc gulpfile.mjs rollup.config.mjs ./

package.json

@@ -73,5 +73,6 @@
   "prettier": {
     "arrowParens": "avoid",
     "singleQuote": true
-  }
+  },
+  "packageManager": "yarn@3.6.1"
 }