Explain where the beeper-line container image comes from

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/configuring-playbook-bridge-beeper-line.md

@@ -36,7 +36,11 @@ matrix_bridge_beeper_line_enabled: true
 
 ### Using another container image
 
-The playbook uses `docker.io/crispyduck/beeper-line` by default. To use an image that you have built and published elsewhere, override the complete image name:
+Upstream does not publish container images publicly, as its CI only pushes them to Beeper's private registry. The playbook therefore uses [`docker.io/crispyduck/beeper-line`](https://hub.docker.com/r/crispyduck/beeper-line) by default, which is a community-maintained image built from the upstream source.
+
+That image is only available for the amd64 architecture. On other architectures, the playbook self-builds the image from source automatically.
+
+To use an image that you have built and published elsewhere, override the complete image name:
 
 ```yaml
 matrix_bridge_beeper_line_container_image_self_build: false

roles/custom/matrix-bridge-beeper-line/defaults/main.yml

@@ -20,7 +20,8 @@ matrix_bridge_beeper_line_container_image_self_build_branch: "{{ 'main' if matri
 # A side effect is that Renovate cannot track updates for this component, so its version needs to be bumped manually.
 matrix_bridge_beeper_line_version: 412eda7f028f0ee946d6fe584c64c7e97f27c7a7
 
-# This image is maintained separately from the upstream project.
+# Upstream does not publish container images publicly (its CI only pushes to Beeper's private registry), so this is a community-maintained image built from the upstream source.
+# It is only available for the amd64 architecture, which is why the playbook falls back to self-building on other architectures.
 # To use another prebuilt image, override this variable directly.
 matrix_bridge_beeper_line_container_image: "{{ matrix_bridge_beeper_line_container_image_registry_prefix }}crispyduck/beeper-line:{{ matrix_bridge_beeper_line_version }}"
 matrix_bridge_beeper_line_container_image_registry_prefix: "{{ 'localhost/' if matrix_bridge_beeper_line_container_image_self_build else matrix_bridge_beeper_line_container_image_registry_prefix_upstream }}"