refactor(infra): bake config via dockerfile_inline, drop Dockerfile files (refs #30)

Replaces the three standalone Dockerfiles (openzaak, opennotificaties,
keycloak) with `build.dockerfile_inline` recipes in the compose files, so the
config bake has no separate Dockerfile artifacts to maintain. Behaviour is
identical: each derived image still COPYies its config in.

- oz-init / keycloak / flowable-init: 2-line inline Dockerfiles.
- Open Notificaties needs no bake at all now — nrc-init runs migrations only,
  so all NRC services use the plain base image (removes a whole derived image).

Why dockerfile_inline and not `docker cp` into named volumes: docker cp avoids
images entirely but needs `docker compose create`, which podman-compose (the
local dev runtime) does not implement — it would break `make openzaak-up` etc.
locally. dockerfile_inline works on both podman-compose and the CI runner
(verified both: oz-init + keycloak inline builds locally; flowable-init inline
has been green on CI since run 27).

Docs updated: gitea-actions-gotchas.md and openzaak.md.

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

infra/opennotificaties/docker-compose.yml

@@ -33,12 +33,8 @@ services:
     networks: [cg]
 
   nrc-init:
-    build:
-      context: .
-      dockerfile: Dockerfile
-      args:
-        OPENNOTIFICATIES_TAG: ${OPENNOTIFICATIES_TAG:-1.16.1}
-    image: register-referentie/opennotificaties:dev
+    # Plain base image — nrc-init runs migrations only (see command below).
+    image: docker.io/openzaak/open-notificaties:${OPENNOTIFICATIES_TAG:-1.16.1}
     environment: &nrc-env
       DJANGO_SETTINGS_MODULE: nrc.conf.docker
       SECRET_KEY: ${NRC_SECRET_KEY:-dev-only-not-for-production}
@@ -69,7 +65,7 @@ services:
     networks: [cg]
 
   nrc-web:
-    image: register-referentie/opennotificaties:dev
+    image: docker.io/openzaak/open-notificaties:${OPENNOTIFICATIES_TAG:-1.16.1}
     environment: *nrc-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)"]
@@ -85,7 +81,7 @@ services:
     networks: [cg]
 
   nrc-celery:
-    image: register-referentie/opennotificaties:dev
+    image: docker.io/openzaak/open-notificaties:${OPENNOTIFICATIES_TAG:-1.16.1}
     environment: *nrc-env
     command: /celery_worker.sh
     depends_on: