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>

infra/flowable/docker-compose.yml

@@ -38,16 +38,11 @@ services:
   # Deploys workflows/registratie.bpmn via the REST API once flowable-rest is up.
   # Idempotent: skips if a deployment named "registratie" already exists.
   flowable-init:
-    # The BPMN is baked into a tiny curl image (build context = repo-root
-    # workflows/) instead of bind-mounted, so the deploy works on Gitea's
-    # containerized runner too. See docs/runbooks/gitea-actions-gotchas.md.
-    build:
-      context: ../../workflows
-      dockerfile_inline: |
-        FROM docker.io/curlimages/curl:latest
-        COPY registratie.bpmn /work/registratie.bpmn
-    image: register-referentie/flowable-init:dev
+    image: docker.io/curlimages/curl:latest
     restart: "no"
+    # registratie.bpmn is streamed into this external volume by infra/seed-config.sh.
+    volumes:
+      - fl-bpmn:/work:ro
     command:
       - sh
       - -c
@@ -66,6 +61,10 @@ services:
 
 volumes:
   flowable-db:
+  # populated out-of-band by infra/seed-config.sh (docker cp) — see that script.
+  fl-bpmn:
+    external: true
+    name: rr-fl-bpmn
 
 networks:
   cg: