diff --git a/.github/list-subimages b/.github/list-subimages
index 5d0e5ef..7966019 100755
--- a/.github/list-subimages
+++ b/.github/list-subimages
@@ -3,6 +3,8 @@ import glob
 import re
 import sys
 import argparse
+import json
+
 
 all_subimages = {re.sub(r"^Dockerfile-", "", dockerfile) for dockerfile in glob.glob("Dockerfile-*")}
 core_subimages = {si for si in all_subimages if si.startswith("core")}
@@ -15,15 +17,15 @@ parser.add_argument('--core', action='store_true',
                     default=False, help='List core subimages')
 parser.add_argument('--rest', action='store_true',
                     default=False, help='List rest subimages')
-parser.add_argument('--csv', action='store_true',
-                    default=False, help='Return list as CSV')
+parser.add_argument('--json', action='store_true',
+                    default=False, help='Return list as JSON')
 args = parser.parse_args()
 
 
 def list_(subimages):
     subimages = sorted(subimages)
-    if args.csv:
-        print(",".join(subimages))
+    if args.json:
+        print(json.dumps(subimages))
     else:
         print("\n".join(subimages))
 
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 8c7017d..52f4ea4 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -10,7 +10,7 @@ on:
       - 'test/github-actions'
 
 jobs:
-  generate-matrix:
+  matrix:
     runs-on: ubuntu-latest
     outputs:
       core: ${{ steps.step1.outputs.core }}
@@ -23,13 +23,14 @@ jobs:
         name: Generate outputs
         id: step1
         run: |
-          echo "core=$(./.github/list-subimages --core --csv)" >>$GITHUB_OUTPUT
-          echo "rest=$(./.github/list-subimages --rest --csv)" >>$GITHUB_OUTPUT
+          echo "core=$(./.github/list-subimages --core --json)" >>$GITHUB_OUTPUT
+          echo "rest=$(./.github/list-subimages --rest --json)" >>$GITHUB_OUTPUT
 
   build-core:
+    needs: matrix
     strategy:
       matrix:
-        subimage: ["core"]
+        subimage: ${{ fromJson(needs.matrix.outputs.core) }}
     uses: ./.github/workflows/build-subimage.yml
     with:
       subimage: ${{ matrix.subimage }}