chore(deps): update registry.suse.com/bci/dotnet-sdk docker tag to v8.0.21

Author: renovate[bot]

.github/workflows/build-image.yml

@@ -1,20 +1,42 @@
 name: Build image(s) on PR
 
 on:
-  pull_request:
+  push:
     paths:
       - ci/images/Dockerfile.*
 
 permissions: {}
 
 jobs:
+  detect-changes:
+    if: github.repository == 'CycloneDX/cdxgen'
+    runs-on: ubuntu-24.04
+    outputs:
+      imagefiles: ${{ steps.changed.outputs.files }}
+    steps:
+      - name: Get changed files
+        id: changed
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const response = await github.rest.repos.compareCommits({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              base: context.payload.before,
+              head: context.payload.after
+            });
+            const files = response.data.files
+              .map(f => f.filename)
+              .filter(f => f.includes("Dockerfile"));
+            core.setOutput("files", JSON.stringify(files));
   image:
     if: github.repository == 'CycloneDX/cdxgen'
+    needs: detect-changes
     strategy:
       fail-fast: false
       matrix:
-        imagefile: ${{ github.event.head_commit.modified }}
-        runner: [ubuntu-24.04, ubuntu-24.04-arm]
+        imagefile: ${{ fromJson(needs.detect-changes.outputs.matrix) }}
+        runner: [ ubuntu-24.04, ubuntu-24.04-arm ]
         include:
           - runner: ubuntu-24.04
             platform: linux/amd64

ci/images/Dockerfile.dotnet8

@@ -1,5 +1,5 @@
 # Base-image
-FROM registry.suse.com/bci/dotnet-sdk:8.0.20@sha256:255e01937ff1b7626e75ec87cf17a60b8e86437fb123f66e2c2f9c672bcd9309 AS base
+FROM registry.suse.com/bci/dotnet-sdk:8.0.21@sha256:980f77d6eb57252d9d486651f22aa4674a476c5a3b2dfd35b8322fb4c008cda6 AS base
 
 ARG GITHUB_URL=https://github.com
 ARG NODEJS_DIST_URL