Use BuildKit for ma1sd Docker building

Newer versions (`master`) use things like `--platform=...`,
which are not supported unless we enable the new BuildKit building
backend.

roles/matrix-ma1sd/tasks/setup_install.yml

@@ -84,7 +84,7 @@
     register: matrix_ma1sd_git_pull_results
 
   - name: Ensure ma1sd Docker image is built
-    shell: "./gradlew dockerBuild"
+    shell: "DOCKER_BUILDKIT=1 ./gradlew dockerBuild"
     args:
       chdir: "{{ matrix_ma1sd_docker_src_files_path }}"