image history concepts doc (#55)

Concepts doc for image history feature.

---

### **Checklist**
- [ ] Tests added/updated
- [x] Documentation updated (if needed)
- [x] Code conforms to style guidelines

docs/_data/navigation.yml

@@ -66,6 +66,8 @@
       link: "/azure-linux-image-tools/imagecustomizer/concepts/pxe"
     - name: Verity
       link: "/azure-linux-image-tools/imagecustomizer/concepts/verity"
+    - name: Image History 
+      link: "/azure-linux-image-tools/imagecustomizer/concepts/imagehistory"
 - name: How-To
   subnav:
     - name: Running in a container

docs/imagecustomizer/api/configuration/os.md

@@ -119,6 +119,8 @@ Options for configuring image history.
 
 Set value to `none` to disable.
 
+To learn more about image history, refer to the [Image History Concept](../../concepts/imagehistory.md) documentation.
+
 ```yaml
 os:
   imageHistory: none

docs/imagecustomizer/concepts/imagehistory.md

@@ -0,0 +1,136 @@
+# Image History
+
+The Image History feature records the customization history of images in a
+structured JSON format. It enables users to:
+
+- Trace the lineage of customized images.
+- Verify input integrity using SHA256 hashes.
+- Debug and identify issues in customization workflows.
+
+Image history tracking can be disabled by explicitly configuring the option in
+the [OS configuration settings](../api/configuration/os.md)
+
+## History Storage
+
+- **Location:** `/usr/share/image-customizer/history.json`.
+- **Format:** An array of JSON objects, each representing a customization step.
+- **Order:** Each new history entry is appended to the end of the existing list
+  within the history file, preserving chronological sequence across image
+  customizations.
+
+## Metadata Captured
+
+Each entry in the history includes:
+
+- **Timestamp:** Time of customization.
+- **Tool Version:** Image customizer version used.
+- **Image UUID:** Unique identifier for the image.
+- **Configuration:** Captures the config used for customization (with certain
+  modifications).
+
+## Config Modifications
+
+The configuration undergoes specific modifications before being serialized into
+JSON format:
+
+**Hashing:**
+
+- SHA256 hashes are generated for scripts, additional files, and the contents of
+  additional directories to ensure input integrity.
+- If only content is provided for scripts or additional files (and no source
+  path), no hash is calculated or added. When a source path is specified, a
+  SHA256 hash is computed and included as a field in the JSON configuration.
+- For additional directories, a hashmap of each file to its SHA256 hash is added
+  as a field in the JSON configuration.
+
+**Redaction:** Known sensitive information, such as SSH public keys, is replaced
+with `[redacted]` to maintain security.
+
+## JSON Schema
+
+```json
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "title": "ImageHistoryList",
+  "type": "array",
+  "items": {
+    "type": "object",
+    "properties": {
+      "timestamp": {
+        "type": "string",
+        "format": "date-time",
+        "description": "The timestamp for when the image was built."
+      },
+      "toolVersion": {
+        "type": "string",
+        "description": "The version of the tool used for the customization."
+      },
+      "imageUuid": {
+        "type": "string",
+        "description": "A unique identifier for the customized image."
+      },
+      "config": {
+        "type": "object",
+        "description": "Configuration data with added/redacted fields."
+      }
+    },
+    "required": ["timestamp", "toolVersion", "imageUuid", "config"]
+  }
+}
+```
+
+## Example JSON Array
+
+```json
+[
+  {
+    "timestamp": "2024-12-09T17:20:54Z",
+    "toolVersion": "0.1.0",
+    "imageUuid": "4a0cb56c-efa8-6636-528f-033477a7bb27",
+    "config": {
+      "additionalFiles": [
+        {
+          "destination": "/a.txt",
+          "source": "files/a.txt",
+          "sha256hash": "06577bd4a35a3fb866f891567b5a9ff67223c2f4422fb7629836d0cadb603ed3"
+        }
+      ],
+      "additionalDirs": [
+        {
+          "source": "dirs/a",
+          "destination": "/",
+          "sha256hashmap": {
+            "usr/local/bin/animals.sh": "13115588883d6f8960cf2e5f03ffcd73babcbb8da69789683715911487c8b1b6"
+          }
+        }
+      ],
+      "users": [
+        {
+          "name": "test",
+          "sshPublicKeys": "[redacted]"
+        }
+      ]
+    }
+  },
+  {
+    "timestamp": "2024-12-10T12:34:56Z",
+    "toolVersion": "0.2.0",
+    "imageUuid": "5b6c7d8e-9f10-1234-5678-abcdefabcdef",
+    "config": {
+      "additionalFiles": [
+        {
+          "destination": "/b.txt",
+          "source": "files/b.txt",
+          "sha256hash": "13579bd4a35a3fb866f891567b5a9ff67223c2f4422fb7629836d0cadb603ee4"
+        }
+      ],
+      "users": [
+        {
+          "name": "admin",
+          "sshPublicKeys": "[redacted]"
+        }
+      ]
+    }
+  }
+]
+```