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/openzaak/docker-compose.yml

@@ -32,14 +32,7 @@ services:
     networks: [cg]
 
   oz-init:
-    # Derived image: base OpenZaak + setup_configuration baked in via an inline
-    # Dockerfile. See docs/runbooks/gitea-actions-gotchas.md for why not a mount.
-    build:
-      context: .
-      dockerfile_inline: |
-        FROM docker.io/openzaak/open-zaak:${OPENZAAK_TAG:-1.28.2}
-        COPY setup_configuration /app/setup_configuration
-    image: register-referentie/openzaak:dev
+    image: docker.io/openzaak/open-zaak:${OPENZAAK_TAG:-1.28.2}
     environment: &oz-env
       DJANGO_SETTINGS_MODULE: openzaak.conf.docker
       SECRET_KEY: ${OZ_SECRET_KEY:-dev-only-not-for-production}
@@ -62,6 +55,9 @@ services:
       OPENZAAK_SUPERUSER_EMAIL: admin@localhost
       RUN_SETUP_CONFIG: "true"
     command: /setup_configuration.sh
+    # data.yaml is streamed into this external volume by infra/seed-config.sh.
+    volumes:
+      - oz-config:/app/setup_configuration:ro
     depends_on:
       oz-db:
         condition: service_healthy
@@ -70,7 +66,7 @@ services:
     networks: [cg]
 
   openzaak:
-    image: register-referentie/openzaak:dev
+    image: docker.io/openzaak/open-zaak:${OPENZAAK_TAG:-1.28.2}
     environment: *oz-env
     healthcheck:
       test: ["CMD", "python", "-c", "import requests,sys; sys.exit(0 if requests.head('http://localhost:8000/admin/').status_code in (200,302) else 1)"]
@@ -86,7 +82,7 @@ services:
     networks: [cg]
 
   oz-celery:
-    image: register-referentie/openzaak:dev
+    image: docker.io/openzaak/open-zaak:${OPENZAAK_TAG:-1.28.2}
     environment: *oz-env
     command: /celery_worker.sh
     depends_on:
@@ -96,6 +92,10 @@ services:
 
 volumes:
   oz-db:
+  # populated out-of-band by infra/seed-config.sh (docker cp) — see that script.
+  oz-config:
+    external: true
+    name: rr-oz-config
 
 networks:
   cg: