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] <[email protected]>

Author: dependabot[bot]

.github/workflows/ci.yaml

@@ -11,15 +11,15 @@ jobs:
     name: Test
     runs-on: [self-hosted, linux]
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - name: Run tests
         run: docker build --target test .
   build:
     name: Build
     runs-on: [self-hosted, linux]
     needs: test
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: secondlife/action-build-docker@v1
         with:
           image: platform/nginx-proxy