fix(ci): prevent stale main image publishes

.github/workflows/publish-main.yml

@@ -12,10 +12,6 @@ env:
   DOCKERHUB_IMAGE_NAME: ${{ secrets.DOCKER_USERNAME }}/${{ github.event.repository.name }}
   GITHUB_IMAGE_NAME: ${{ github.repository }}
 
-concurrency:
-  group: main-docker-${{ github.ref }}
-  cancel-in-progress: false
-
 jobs:
   build-and-push-main:
     name: Build and push main Docker images (with docs)
@@ -23,6 +19,9 @@ jobs:
       fail-fast: false
       matrix:
         variant: [debian, alpine]
+    concurrency:
+      group: main-docker-${{ github.ref }}-${{ matrix.variant }}
+      cancel-in-progress: true
     runs-on: ubuntu-latest
     permissions:
       contents: read