build: Dockerfiles generated by build.py for RHEL require base image specification (#7970)

Author: saturley-hall

build.py

@@ -1643,8 +1643,14 @@ def create_build_dockerfiles(
 ):
     if "base" in images:
         base_image = images["base"]
+        if target_platform() == "rhel":
+            print(
+                "warning: RHEL is not an officially supported target and you will probably experience errors attempting to build this container."
+            )
     elif target_platform() == "windows":
         base_image = "mcr.microsoft.com/dotnet/framework/sdk:4.8"
+    elif target_platform() == "rhel":
+        raise KeyError("A base image must be specified when targeting RHEL")
     elif FLAGS.enable_gpu:
         base_image = "nvcr.io/nvidia/tritonserver:{}-py3-min".format(
             FLAGS.upstream_container_version