[ericof]

This adds -demo variants for the 4.3, 5.0, 5.1 and 5.2 series. Each one is the official plone image for its series (4.3.19, 5.0.8, 5.1.6, 5.2.14), pinned by digest, plus a database with a Plone site already created at /Plone. The base images are not rebuilt (see #213).

How it works (demo/):

• The official images ship no curl, and nothing can be installed into them, so seed.sh creates the site from the host. It starts the official image, runs legacy/shared/create-plone-site.sh, stops Zope, checks that the storage was closed cleanly, and copies Data.fs and the blobs out with docker cp.
• Dockerfile adds that seed at /app/seed, plus a wrapper entrypoint. On first start the wrapper copies the seed into /data, then runs the base image's own entrypoint. When /data already holds a database, nothing is copied.
• The demo build workflow seeds, builds and smoke-tests all four series. On main it pushes <series>-demo and <full>-demo to plone/plone and GHCR, like the legacy workflow.

What differs between the base images: 5.2 only stops cleanly on SIGINT. With SIGTERM, docker stop ran into its 120s timeout and killed Zope. 5.2 also logs to stdout/stderr instead of /data/log/.

Changes to the shared legacy scripts. These make the legacy CI rebuild all 11 series on this PR:

• create-plone-site.sh: on Plone 5, the theme check now looks for ++theme++barceloneta in the rendered page. The skin check used for Plone 4 found nothing on Plone 5.
• smoke-test.sh: new SMOKE_LOG_SOURCE=docker option, which reads product-load errors from docker logs.
• A gate bug: the JSON check passed on 5.2 even though the probe never ran. The probe was Python 2 only and failed to compile, and the check matched the marker inside the traceback that quoted the probe's source. The probe now runs on Python 2.3 – 3.8, and the check only accepts the marker at the start of a line.

Tested locally:

• All four -demo images pass the smoke test: the site is already there on first start, admin:admin authenticates, and the product-load and JSON checks pass.
• The old probe now fails the fixed check.
• The new probe passes on 9 images, from legacy 1.0 (Python 2.3) to 5.2 (Python 3.8).

After merge: add the demo subsections for 4.3, 5.0, 5.1 and 5.2 to plone/documentation's docs/install/containers/images/historical.md (plone/documentation#2103).

Refs #211