Moves matrix_dendrite_container_image_self_build

to a conditional block, similar to synapse. Also adds the
DOCKER_BUILDKIT=1 explicitly to ensure buildkit is used.

roles/custom/matrix-dendrite/tasks/setup_install.yml

@@ -47,17 +47,6 @@
   delay: "{{ devture_playbook_help_container_retries_delay }}"
   until: result is not failed
 
-- name: Ensure Dendrite repository is present on self-build
-  ansible.builtin.git:
-    repo: "{{ matrix_dendrite_container_image_self_build_repo }}"
-    dest: "{{ matrix_dendrite_docker_src_files_path }}"
-    version: "{{ matrix_dendrite_docker_image.split(':')[1] }}"
-    force: "yes"
-  become: true
-  become_user: "{{ matrix_user_username }}"
-  register: matrix_dendrite_git_pull_results
-  when: "matrix_dendrite_container_image_self_build | bool"
-
 # We do this so that the signing key would get generated.
 # We don't use the `docker_container` module, because using it with `cap_drop` requires
 # a very recent version, which is not available for a lot of people yet.
@@ -88,10 +77,32 @@
     owner: "{{ matrix_user_username }}"
     group: "{{ matrix_user_groupname }}"
 
+- when: "matrix_dendrite_container_image_self_build | bool"
+  block:
+    - name: Ensure Dendrite repository is present on self-build
+      ansible.builtin.git:
+        repo: "{{ matrix_dendrite_container_image_self_build_repo }}"
+        dest: "{{ matrix_dendrite_docker_src_files_path }}"
+        version: "{{ matrix_dendrite_docker_image.split(':')[1] }}"
+        force: "yes"
+      become: true
+      become_user: "{{ matrix_user_username }}"
+      register: matrix_dendrite_git_pull_results
+
+    - name: Check if Dendrite Docker image exists
+      ansible.builtin.command: "{{ devture_systemd_docker_base_host_command_docker }} images --quiet --filter 'reference={{ matrix_dendrite_docker_image }}'"
+      register: matrix_dendrite_docker_image_check_result
+      changed_when: false
+
+    # 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 Dendrite Docker image is built
       ansible.builtin.command:
         cmd: "{{ devture_systemd_docker_base_host_command_docker }} build -t {{ matrix_dendrite_docker_image }} {{ matrix_dendrite_docker_src_files_path }}"
-  when: "matrix_dendrite_container_image_self_build | bool"
+      environment:
+        DOCKER_BUILDKIT: 1
+      when: "matrix_dendrite_git_pull_results.changed | bool or matrix_dendrite_docker_image_check_result.stdout == ''"
 
 - name: Ensure Dendrite container network is created
   community.general.docker_network: