Bump actions/checkout from 3 to 4

Bumps [actions/checkout](https://github.com/actions/checkout) from 3 to 4.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v3...v4)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

.github/workflows/dockerimage.yml

@@ -11,7 +11,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
 
     # https://github.com/FiloSottile/mkcert#installation
     - name: Install and setup mkcert

.github/workflows/push-to-ghcr.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: "ubuntu-20.04"
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
 
       # https://github.com/marketplace/actions/push-to-ghcr
       - name: Build and publish a Docker image for macbre/nginx-http3