[Docker] Explain how to update tagging lambda (ray-project#24862)

Explains how to replace the existing lambda when the code changes.

docker/retag-lambda/README.md

@@ -1,5 +1,17 @@
-Get Docker Retag via wget:
+# Updating this Lambda Function
+
+1. Get Docker Retag via wget:
 ```
+pushd docker/retag-lambda
 wget -q https://github.com/joshdk/docker-retag/releases/download/0.0.2/docker-retag
+popd
 ```
-Package this full library and upload as an AWS Lambda :) 
+
+2. Package this folder:
+```
+pushd docker/retag-lambda
+zip retag-lambda.zip *
+```
+
+3. Head to the AWS Management console & select the `DockerTagLatest` function. Select `Upload from`, then `.zip file` and then select the zip file created in Step 2.
+