refactor(infra): use upstream images verbatim, seed config via docker cp (refs #30)

Drops the inline-build images for the upstream services. The compose now
references the published images directly (openzaak/open-zaak,
openzaak/open-notificaties, keycloak, curl, flowable-rest) with no build for
them, and the config they need is streamed into external named volumes by
infra/seed-config.sh:

  rr-oz-config  -> oz-init     /app/setup_configuration   (data.yaml)
  rr-kc-realms  -> keycloak    /opt/keycloak/data/import   (realm exports)
  rr-fl-bpmn    -> flowable-init /work                     (registratie.bpmn)

How: the seeder creates each volume, `docker create`s a throwaway helper that
mounts it, `docker cp`s the files in, and removes it. docker cp streams over the
Docker API, so it works in Docker-in-Docker (the CI runner) where bind mounts
mount empty. It uses plain `docker create`/`cp` — NOT `docker compose create`,
which podman-compose (local dev) lacks. `external: true` fixed names keep the
volumes identical across docker compose and podman-compose.

Consequence: bare `docker compose up` no longer self-seeds, so use `make up`
(seeds then starts). Every `*-up` target seeds first; `*-down` removes the
external volume. acl/bff are still built (they're our apps, not upstream images).

Verified end-to-end on podman-compose: `make keycloak-up` seeds rr-kc-realms,
the upstream Keycloak mounts it, and --import-realm imports all four realms
(digid realm returns 200). Seeder runs in ~2s.

Docs updated: gitea-actions-gotchas.md, ci.md, openzaak.md.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/runbooks/openzaak.md

@@ -73,7 +73,10 @@ The Makefile auto-points `DOCKER_HOST` at the Podman socket when it exists, so t
   resultaattypen — beyond the lean seed). List with `?status=alles`.
 - **Image tag.** Pinned to `openzaak/open-zaak:1.28.2` via `${OPENZAAK_TAG}` (bump
   deliberately, not via `:latest`).
-- **Config is baked, not mounted.** `setup_configuration/data.yaml` is `COPY`-ed into
-  a derived image via `oz-init`'s inline Dockerfile (`build.dockerfile_inline` in
-  the compose) rather than bind-mounted, so the init container finds it on Gitea's
-  containerized CI runner too. See [gitea-actions-gotchas.md](gitea-actions-gotchas.md).
+- **Config arrives via a volume, not a bind mount.** `setup_configuration/data.yaml`
+  is streamed into the external `rr-oz-config` volume by `infra/seed-config.sh`
+  (`docker cp`) and mounted at `/app/setup_configuration`, so the init container
+  finds it on Gitea's containerized CI runner too (bind mounts don't reach sibling
+  containers there). The image is the upstream `openzaak/open-zaak` verbatim — no
+  build. Run via `make openzaak-up` (seeds first). See
+  [gitea-actions-gotchas.md](gitea-actions-gotchas.md).