Add comment describing the aws cli query

Co-authored-by: Paula <[email protected]>

hooks/environment

@@ -157,7 +157,13 @@ function login() {
 }
 
 function assume_role_for_ecr_login() {
-  local export_credentials; export_credentials="$(aws sts assume-role \
+  local export_credentials
+
+  # This query creates an outer array, then multiple inner arrays of [key,value]
+  # pairs. Then it projects the outer array, and joins each inner array to form
+  # a key=value string. Printing the final array with --output text results in a
+  # string of the key=value pairs joined by space characters.
+  export_credentials="$(aws sts assume-role \
     --role-arn "${BUILDKITE_PLUGIN_ECR_ASSUME_ROLE_ROLE_ARN}" \
     --role-session-name "ecr-login-buildkite-plugin" \
     --duration-seconds "${BUILDKITE_PLUGIN_ECR_ASSUME_ROLE_DURATION_SECONDS:-3600}" \