refactor(setup-db): to run trivy in docker (#54)

As part of our safety initiative, I'm refactoring setup-db to run trivy inside a Docker container with minimal privileges, reducing leakage of secrets, files, etc. to a minimum in case the dependency gets compromised.
Additionally, we are always pinning the trivy docker image to a fixed digest. Renovate has been configured to keep the Trivy image version up-to-date.

Reviewed-on: #54
Reviewed-by: branch-buddy <branch-buddy@t00n.de>
Co-authored-by: Timo Behrendt <t.behrendt@t00n.de>
Co-committed-by: Timo Behrendt <t.behrendt@t00n.de>

renovate.json

@@ -4,6 +4,17 @@
     "local>t.behrendt/renovate-configs:common",
     "local>t.behrendt/renovate-configs:action"
   ],
+  "customManagers": [
+    {
+      "customType": "regex",
+      "description": "Update Trivy docker image",
+      "managerFilePatterns": ["/(^|/)setup-db/action\\.ya?ml$/"],
+      "datasourceTemplate": "docker",
+      "matchStrings": [
+        "default:\\s*\"(?<depName>ghcr\\.io/aquasecurity/trivy):(?<currentValue>[\\d\\.]+)(?:@(?<currentDigest>sha256:[a-f0-9]+))?\""
+      ]
+    }
+  ],
   "packageRules": [
     {
       "matchManagers": ["dockerfile", "kubernetes", "helmfile", "helm-values"],

setup-db/README.md

@@ -2,7 +2,9 @@
 
 A reusable Gitea Action that sets up the Trivy vulnerability database, restoring from cache if available.
 
-**Note:** This action only sets up the database. Trivy itself must be installed separately (e.g., using the `setup-trivy` action).
+The action runs Trivy inside **Docker** with a restricted container configuration so the Trivy runtime is isolated from the host while the database is downloaded into your cache directory.
+
+**Note:** This action only prepares the vulnerability database. If you run Trivy on the runner host for scans (for example `trivy fs .`), install Trivy separately (e.g. with a `setup-trivy` action or your own step).
 
 ## Usage
 
@@ -37,5 +39,8 @@ jobs:
 ## Inputs
 
 | Input            | Description                                                                 | Required | Default |
-| ----------- | --------------------------------- | -------- | ---------------- |
-| `cache-dir` | Path to the Trivy cache directory | No       | `~/.cache/trivy` |
+| ---------------- | --------------------------------------------------------------------------- | -------- | ------- |
+| `cache-dir`      | Path to the Trivy cache directory                                           | No       | `${{ runner.temp }}/trivy` |
+| `trivy-version`  | Docker image reference for Trivy (digest pin recommended)                   | No       | Pinned `ghcr.io/aquasecurity/trivy` image in `action.yaml` |
+
+**`trivy-version` is optional.** If you omit it, the action uses the default image (version and digest) from `action.yaml`. Set it only when you need a different Trivy image or your own digest pin.