update readme with private registry section (#610)

README.md

@@ -1,5 +1,5 @@
 <p align="center">
-    <img src="https://user-images.githubusercontent.com/5199289/136844524-1527b09f-c5cb-4aa9-be54-5aa92a6086c1.png" width="271">
+    <img src="https://user-images.githubusercontent.com/5199289/136844524-1527b09f-c5cb-4aa9-be54-5aa92a6086c1.png" width="271" alt="Cute pink owl syft logo">
 </p>
 
 [![Validations](https://github.com/anchore/syft/actions/workflows/validations.yaml/badge.svg)](https://github.com/anchore/syft/actions/workflows/validations.yaml)
@@ -94,7 +94,7 @@ The output format for Syft is configurable as well:
 syft packages <image> -o <format>
 ```
 
-Where the `format`s available are:
+Where the `formats` available are:
 - `json`: Use this to get as much information out of Syft as possible!
 - `text`: A row-oriented, human-and-machine-friendly output.
 - `cyclonedx`: A XML report conforming to the [CycloneDX 1.2 specification](https://cyclonedx.org/specification/overview/).
@@ -102,6 +102,88 @@ Where the `format`s available are:
 - `spdx-json`: A JSON report conforming to the [SPDX 2.2 JSON Schema](https://github.com/spdx/spdx-spec/blob/v2.2/schemas/spdx-schema.json).
 - `table`: A columnar summary (default).
 
+## Private Registry Authentication
+
+### Local Docker Credentials
+When a container runtime is not present, Syft can still utilize credentials configured in common credential sources (such as `~/.docker/config.json`). 
+It will pull images from private registries using these credentials. The config file is where your credentials are stored when authenticating with private registries via some command like `docker login`. 
+For more information see the `go-containerregistry` [documentation](https://github.com/google/go-containerregistry/tree/main/pkg/authn).
+
+
+An example `config.json` looks something like this:
+```
+// config.json
+{
+	"auths": {
+		"registry.example.com": {
+			"username": "AzureDiamond",
+			"password": "hunter2"
+		}
+	}
+}
+```
+
+You can run the following command as an example. It details the mount/environment configuration a container needs to access a private registry:
+
+`docker run -v ./config.json:/config/config.json -e "DOCKER_CONFIG=/config" anchore/syft:latest  <private_image>`
+
+
+### Docker Credentials in Kubernetes
+The below section shows a simple workflow on how to mount this config file as a secret into a container on kubernetes.
+1. Create a secret. The value of `config.json` is important. It refers to the specification detailed [here](https://github.com/google/go-containerregistry/tree/main/pkg/authn#the-config-file). 
+Below this section is the `secret.yaml` file that the pod configuration will consume as a volume. 
+The key `config.json` is important. It will end up being the name of the file when mounted into the pod.
+    ```
+    # secret.yaml
+    
+    apiVersion: v1
+    kind: Secret
+    metadata:
+      name: registry-config
+      namespace: syft
+    data:
+      config.json: <base64 encoded config.json>
+    ```
+
+    `kubectl apply -f secret.yaml`
+
+
+2. Create your pod running syft. The env `DOCKER_CONFIG` is important because it advertises where to look for the credential file. 
+In the below example, setting `DOCKER_CONFIG=/config` informs syft that credentials can be found at `/config/config.json`. 
+This is why we used `config.json` as the key for our secret. When mounted into containers the secrets' key is used as the filename. 
+The `volumeMounts` section mounts our secret to `/config`. The `volumes` section names our volume and leverages the secret we created in step one.
+    ```
+    # pod.yaml
+    
+    apiVersion: v1
+    kind: Pod
+    spec:
+      containers:
+        - image: anchore/syft:latest
+          name: syft-private-registry-demo
+          env:
+            - name: DOCKER_CONFIG
+              value: /config
+          volumeMounts:
+          - mountPath: /config
+            name: registry-config
+            readOnly: true
+          args:
+            - <private_image>
+      volumes:
+      - name: registry-config
+        secret:
+          secretName: registry-config
+    ```
+
+    `kubectl apply -f pod.yaml`
+
+
+3. The user can now run `kubectl logs syft-private-registry-demo`. The logs should show the syft analysis for the `<private_image>` provided in the pod configuration.
+
+Using the above information, users should be able to configure private registry access without having to do so in the `grype` or `syft` configuration files.
+They will also not be dependent on a docker daemon, (or some other runtime software) for registry configuration and access.
+
 ## Configuration
 
 Configuration search paths: