Use Docker BuildKit

.github/workflows/build.yml

@@ -7,6 +7,9 @@ on:
   schedule:
     - cron: '0 12 * * *'
 
+env:
+  DOCKER_BUILDKIT: 1
+
 jobs:
   purge_artifacts:
     name: Purge old artifacts

.github/workflows/pr.yml

@@ -4,6 +4,9 @@ on:
     branches:
       - master
 
+env:
+  DOCKER_BUILDKIT: 1
+
 jobs:
   build:
     name: Build ffmpeg