Bump actions/checkout from 4 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 4 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](https://github.com/actions/checkout/compare/v4...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

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

.github/workflows/docker-publish.yml

@@ -33,7 +33,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       # Install the cosign tool except on PR
       # https://github.com/sigstore/cosign-installer

Dockerfile

@@ -13,7 +13,7 @@ RUN if [ "$TARGETPLATFORM" = "linux/amd64" ]; then \
     fi
 
 
-FROM docker.io/restic/restic:0.18.1
+FROM docker.io/restic/restic:0.18.0
 
 RUN apk add --update --no-cache curl mailx shadow