CI: parallelize multiarch build

.github/workflows/docker.yml

@@ -12,6 +12,12 @@ jobs:
     strategy:
       matrix:
         variant: ["client", "server"]
+        platforms: [
+          "linux/amd64",
+          "linux/arm/v6",
+          "linux/arm/v7",
+          "linux/arm64/v8"
+        ]
     steps:
       -
         name: Set up QEMU
@@ -33,4 +39,4 @@ jobs:
           file: ./Dockerfile.${{ matrix.variant }}
           tags: m13253/dns-over-https-${{ matrix.variant }}:latest
           push: true
-          platforms: linux/amd64,linux/arm/v6,linux/arm/v7,linux/arm64/v8
+          platforms: ${{ matrix.platforms }}