Add extra validation based on review.

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

Author: ianwallen

docs/pipelines/agents/docker.md

@@ -319,6 +319,16 @@ Next, create the Dockerfile.
     # Load a token either from the environment variable or by using the service principal credentials.
     load_azp_token() {
       if [ -n "$AZP_CLIENTID" ]; then
+        if [ -z "$AZP_CLIENTSECRET" ]; then
+          echo 1>&2 "error: AZP_CLIENTSECRET must be set when AZP_CLIENTID is used"
+          exit 1
+        fi
+
+        if [ -z "$AZP_TENANTID" ]; then
+          echo 1>&2 "error: AZP_TENANTID must be set when AZP_CLIENTID is used"
+          exit 1
+        fi
+
         echo "Using service principal credentials to get token"
         az login --allow-no-subscriptions --service-principal --username "$AZP_CLIENTID" --password "$AZP_CLIENTSECRET" --tenant "$AZP_TENANTID"
         # adapted from https://learn.microsoft.com/en-us/azure/databricks/dev-tools/user-aad-token