Commit yarn 4 in the tree and install with --immutable (closes #149)

Follows sneak's ruling at
#149 (comment). This
repository now carries yarn 4.18.1 under .yarn/releases, and whichever
yarn the distribution packages hands the work to it. yarn.lock is
converted once, and script/bootstrap installs with --immutable instead
of stopping on yarn's major version. Node on Alpine is nodejs-current.
Every Dockerfile stage that needs yarn also copies .yarnrc.yml and
.yarn/releases/. The policies and both checklists say so, with the
steps for converting and for a new repository.

Model: opus-5-5

.dockerignore

@@ -55,8 +55,11 @@
 **/[iI][dD]_[eE][dD]25519
 **/[iI][dD]_[eE][dD]25519_[sS][kK]
 
-# Dependencies: restored inside the image, never copied in.
+# Dependencies: restored inside the image, never copied in. yarn's install
+# state and cache stay out, and its release file stays in.
 **/node_modules
+.yarn/*
+!.yarn/releases
 
 # OS metadata.
 **/.DS_Store