Add README for docker-credential-mmds

Signed-off-by: Kern Walster <[email protected]>

Author: Kern--

docker-credential-mmds/README.md

@@ -0,0 +1,87 @@
+# Docker Credential Helper MMDS
+
+Docker Credential Helper MMDS is a [credential helper](https://github.com/docker/docker-credential-helpers) for exposing docker credentials inside a [Firecracker microVM](https://github.com/firecracker-microvm/firecracker) via [MMDS](https://github.com/firecracker-microvm/firecracker/blob/main/docs/mmds/mmds-user-guide.md).
+
+# Building
+
+`docker-credential-mmds` can built with
+```
+make
+```
+
+# Configuration
+
+## Docker inside Firecracker
+When building a firecracker rootfs, place `docker-credential-mmds` on the `PATH` then put the following configuration in `~/.docker/config`
+
+```
+{
+	"credsStore": "mmds"
+}
+```
+
+This configures the Docker daemon running inside the Firecracker microVM to read all credentials from MMDS.
+
+## Credentials from Host
+
+`docker-credential-mmds` reads credentials from MMDS inside the Firecracker microVM, but a cooperating process on the host needs to place credentials into MMDS. The credentials must be placed in MMDS under a key called `docker-credentials` which contains maps of host names to `username` and `password`. 
+
+For example, the following configures credentials for the ECR public gallery and docker hub
+```
+{
+	"docker-credentials": {
+		"public.ecr.aws": {
+			"username": "123456789012",
+			"password": "access_key"
+		},
+		"docker.io": {
+			"username": "user",
+			"password": "pass"
+		}
+	}
+}
+```
+
+### Placing credentials with the Firecracker HTTP API
+One way to put credentials into MMDS is with firecracker's HTTP API.
+
+```
+curl --unix-socket /tmp/firecracker.socket -i \
+    -X PUT "http://localhost/mmds"            \
+    -H "Content-Type: application/json"       \
+    -d '{
+		"docker-credentials": {
+			"public.ecr.aws": {
+				"username": "123456789012",
+				"password": "access_key"
+			},
+			"docker.io": {
+				"username": "user",
+				"password": "pass"
+			}	
+		}
+	}'
+```
+
+### Placing credentials with the Firecracker-go-sdk
+For larger systems, it may be useful to write a full program on the host to enable additional features such as credential refreshing. The [firecracker-go-sdk](https://github.com/firecracker-microvm/firecracker-go-sdk) wraps the firecracker HTTP APIs with go APIs for this purpose.
+
+```
+credentials := `{
+	"docker-credentials": {
+		"public.ecr.aws": {
+			"username": "123456789012",
+			"password": "access_key"
+		},
+		"docker.io": {
+			"username": "user",
+			"password": "pass
+		}
+	}
+}`
+fcClient, _ := client.New("/tmp/firecracker.socket")
+fcClient.SetVMMetadata(ctx, &proto.SetVMMetadataRequest{
+		VMID:     vmID,
+		Metadata: credentials,
+	})
+```