Improve self-building experience (avoid conflict with pullable images)

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/716

This patch makes us use more fully-qualified container image names
(either prefixed with docker.io/ or with localhost/).

The latter happens when self-building is enabled.

We've recently had issues where if an image was removed manually
and the service was restarted (making `docker run` fetch it from Docker Hub, etc.),
we'd end up with a pulled image, even though we're aiming for a self-built one.
Re-running the playbook would then not do a rebuild, because:
- the image with that name already exists (even though it's something
else)
- we sometimes had conditional logic where we'd build only if the git
repo changed

By explicitly changing the name of the images (prefixing with localhost/),
we avoid such confusion and the possibility that we'd automatically pul something
which is not what we expect.

Also, I've removed that condition where building would happen on git
changes only. We now always build (unless an image with that name
already exists). We just force-build when the git repo changes.

roles/matrix-coturn/tasks/setup_coturn.yml

@@ -29,17 +29,19 @@
     dest: "{{ matrix_coturn_docker_src_files_path }}"
     version: "{{ matrix_coturn_docker_image.split(':')[1] }}"
     force: "yes"
+  register: matrix_coturn_git_pull_results
   when: "matrix_coturn_enabled|bool and matrix_coturn_container_image_self_build"
 
 - name: Ensure Coturn Docker image is built
   docker_image:
     name: "{{ matrix_coturn_docker_image }}"
     source: build
+    force_source: "{{ matrix_coturn_git_pull_results.changed }}"
     build:
       dockerfile: Dockerfile
       path: "{{ matrix_coturn_docker_src_files_path }}"
       pull: yes
-  when: "matrix_coturn_enabled|bool and matrix_coturn_container_image_self_build"
+  when: "matrix_coturn_enabled|bool and matrix_coturn_container_image_self_build|bool"
 
 - name: Ensure Coturn configuration path exists
   file: