Fix self-building for Synapse v1.51.0 (requires BuildKit)

Synapse v1.51.0 requires to be built with BuildKit since
matrix-org/synapse#11691

The `docker_image` Ansible module does not support BuildKit
(ansible-collections/community.general#514),
so we had to switch to a `docker build` call.

roles/matrix-synapse/tasks/synapse/setup_install.yml

@@ -18,25 +18,33 @@
     group: "{{ matrix_user_groupname }}"
   when: "not local_path_media_store_stat.failed and not local_path_media_store_stat.stat.exists"
 
-- name: Ensure Synapse repository is present on self-build
-  git:
-    repo: "{{ matrix_synapse_container_image_self_build_repo }}"
-    dest: "{{ matrix_synapse_docker_src_files_path }}"
-    version: "{{ matrix_synapse_docker_image.split(':')[1] }}"
-    force: "yes"
-  register: matrix_synapse_git_pull_results
-  when: "matrix_synapse_container_image_self_build|bool"
+- block:
+  - name: Ensure Synapse repository is present on self-build
+    git:
+      repo: "{{ matrix_synapse_container_image_self_build_repo }}"
+      dest: "{{ matrix_synapse_docker_src_files_path }}"
+      version: "{{ matrix_synapse_docker_image.split(':')[1] }}"
+      force: "yes"
+    register: matrix_synapse_git_pull_results
 
-- name: Ensure Synapse Docker image is built
-  docker_image:
-    name: "{{ matrix_synapse_docker_image }}"
-    source: build
-    force_source: "{{ matrix_synapse_git_pull_results.changed if ansible_version.major > 2 or ansible_version.minor >= 8 else omit }}"
-    force: "{{ omit if ansible_version.major > 2 or ansible_version.minor >= 8 else matrix_synapse_git_pull_results.changed }}"
-    build:
-      dockerfile: docker/Dockerfile
-      path: "{{ matrix_synapse_docker_src_files_path }}"
-      pull: yes
+  - name: Check if Synapse Docker image exists
+    command: "{{ matrix_host_command_docker }} images --quiet --filter 'reference={{ matrix_synapse_docker_image }}'"
+    register: matrix_synapse_docker_image_check_result
+
+  # Invoking the `docker build` command here, instead of calling the `docker_image` Ansible module,
+  # because the latter does not support BuildKit.
+  # See: https://github.com/ansible-collections/community.general/issues/514
+  - name: Ensure Synapse Docker image is built
+    shell:
+      chdir: "{{ matrix_synapse_docker_src_files_path }}"
+      cmd: |
+        {{ matrix_host_command_docker }} build \
+        -t "{{ matrix_synapse_docker_image }}" \
+        -f docker/Dockerfile \
+        .
+    environment:
+      DOCKER_BUILDKIT: 1
+    when: "matrix_synapse_git_pull_results.changed|bool or matrix_synapse_docker_image_check_result.stdout == ''"
   when: "matrix_synapse_container_image_self_build|bool"
 
 - name: Ensure Synapse Docker image is pulled