ci(docker): drop unused pnpm setup that broke setup-node@v5 caching

The Build <function>-fn matrix jobs were failing in "Post Setup
Node.js" with:

    Path Validation Error: Path(s) specified in the action for
    caching do(es) not exist, hence no cache is being saved.

Root cause: pnpm/action-setup@v6 ran before actions/setup-node@v5,
exposing PNPM_HOME. setup-node@v5 then auto-detected pnpm via the
packageManager field in root package.json and enabled pnpm-store
caching — but these jobs never run pnpm install (they only invoke
node --experimental-strip-types scripts/generate.ts). The store
path never gets created, and v5 promotes the missing-path warning
to an error during cache save.

Fix: remove pnpm/action-setup from docker.yaml since the workflow
doesn't use pnpm on the runner. The actual function builds run
inside Docker, which installs pnpm itself via
"npm install -g pnpm@<v>". simple-email had been passing only by
accident (cache hit from a previous warm run); the other three were
deterministic failures.

Author: Anmol1696

.github/workflows/docker.yaml

@@ -56,9 +56,12 @@ jobs:
       - name: Checkout
         uses: actions/checkout@v5
 
-      - name: Setup pnpm
-        uses: pnpm/action-setup@v6
-
+      # No pnpm/action-setup here: this workflow only runs
+      # `node --experimental-strip-types scripts/generate.ts` on the
+      # runner (zero deps); the actual builds happen inside Docker.
+      # Including pnpm makes setup-node@v5 auto-detect it via
+      # packageManager + PNPM_HOME and try to save a cache for an
+      # empty store, which fails with "Path Validation Error".
       - name: Setup Node.js
         uses: actions/setup-node@v5
         with: