Use Node.js 26 and pnpm 12 with Aurora 1.0.0-alpha.20 - #6

Aurora moved to Node.js 26 (plone/aurora#293) and pnpm 12 (plone/aurora#294), both released in 1.0.0-alpha.20. This PR moves the images over too.

Change

• Node.js 26: aurora-builder and aurora-prod-config build from node:26-bookworm-slim instead of node:24-bookworm-slim. The matching comments in .hadolint.yaml and release.yml are updated. Node.js 26 no longer bundles corepack, but the images already install it with npm.
• Aurora 1.0.0-alpha.20: version.txt goes from alpha.19 to alpha.20. pnpm 12 needs alpha.20, because alpha.19 writes @plone/types as workspace: * in @plone/client and pnpm 12 rejects that.
• Cookieplone templates pin: COOKIEPLONE_REPOSITORY_TAG moves to the merge commit of Use pnpm 12.14.0 in the Aurora templates cookieplone-templates#495. That commit includes:
  • Add Node.js 26 to the supported Node.js versions of the Aurora and Volto templates cookieplone-templates#485, without which the template's pre-prompt check rejects Node.js 26;
  • Use pnpm 12.14.0 in the Aurora templates cookieplone-templates#495, which sets the packageManager of the generated project to pnpm@12.14.0.
• pnpm binary in the main image: pnpm 12 ships as a native binary. corepack install only caches the launcher, and corepack downloaded the binary the first time pnpm ran, so every new container fetched it on start. pnpm/Dockerfile now runs pnpm --version during the build, so the binary is already in the image.

Checks

Run locally on linux/arm64:

• make lint passes.
• make build-images builds the builder, dev, prod-config and main images.
• Node.js and pnpm versions:
  • builder, dev and main report Node.js v26.13.0 and pnpm 12.14.0;
  • prod-config has no package.json, so corepack resolves the latest pnpm there, as before.
• The main image runs Aurora 1.0.0-alpha.20. It starts, react-router-serve listens, and the healthcheck reports healthy. pnpm doesn't download anything when the container starts.
• With no Plone backend running, GET / returns 500. I didn't compare this against the current images.

The amd64 builds only run in CI.