chore(deps): update gcr.io/kaniko-project/executor docker tag to v1.20.0 (#30)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| [gcr.io/kaniko-project/executor](https://github.com/GoogleContainerTools/kaniko) | minor | `v1.18.0-debug` -> `v1.20.0-debug` |

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Co-authored-by: fjadmin <fjadmin@demo.rattermeyer.de>
Reviewed-on: #30
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Co-committed-by: Renovate Bot <renovatebot@demo.rattermeyer.de>

.woodpecker/.documentation.yml

@@ -29,7 +29,7 @@ steps:
       - pnpm antora generate --stacktrace antora-playbook.yml
     directory: documentation
   "assemble:documentation":
-    image: gcr.io/kaniko-project/executor:v1.18.0-debug
+    image: gcr.io/kaniko-project/executor:v1.20.0-debug
     commands: # Test if using --insecure can be used to directly push to the registry
       - echo "$CI_WORKSPACE"
       - echo "$PWD"