.github/workflows/build.yaml: use a matrix instead of all at once

it seems there's some metadata leakage between images, this should
ensure that's prevented.

Author: classabbyamp

.github/workflows/build.yaml

@@ -6,6 +6,9 @@ on:
   pull_request:
     branches:
       - master
+    paths-ignore:
+      - COPYING
+      - README.md
   push:
     branches:
       - master
@@ -20,8 +23,17 @@ concurrency:
 jobs:
   build:
     runs-on: ubuntu-latest
-    outputs:
-      metadata: ${{ steps.build_and_push.outputs.metadata }}
+
+    strategy:
+      matrix:
+        libc:
+          - 'glibc'
+          - 'musl'
+        variant:
+          - ''
+          - '-full'
+          - '-busybox'
+
     steps:
       - name: Checkout
         uses: classabbyamp/treeless-checkout-action@v1
@@ -31,12 +43,7 @@ jobs:
         uses: docker/metadata-action@v4
         with:
           images: |
-            ghcr.io/${{ github.repository_owner }}/void-glibc
-            ghcr.io/${{ github.repository_owner }}/void-glibc-full
-            ghcr.io/${{ github.repository_owner }}/void-glibc-busybox
-            ghcr.io/${{ github.repository_owner }}/void-musl
-            ghcr.io/${{ github.repository_owner }}/void-musl-full
-            ghcr.io/${{ github.repository_owner }}/void-musl-busybox
+            ghcr.io/${{ github.repository_owner }}/void-${{ matrix.libc }}${{ matrix.variant }}
           tags: |
             type=sha,prefix=
             type=raw,value=latest,enable={{is_default_branch}}
@@ -70,6 +77,7 @@ jobs:
         uses: docker/bake-action@v3
         with:
           push: ${{ github.event_name != 'pull_request' }}
+          targets: void-${{ matrix.libc }}${{ matrix.variant }}
           files: |
             docker-bake.hcl
             ${{ steps.meta.outputs.bake-file }}