Update CONTAINER_TOOL evaluation in makefile to warn instead of error (#852)

when unable to find docker or podman as this evaluation results in
an error in environments where docker or podman is not installed
even when using targets that do not require a container runtime

Signed-off-by: everettraven <everettraven@gmail.com>

Author: everettraven

Makefile

@@ -50,7 +50,7 @@ CONTAINER_RUNTIME := docker
 else ifneq (, $(shell command -v podman 2>/dev/null))
 CONTAINER_RUNTIME := podman
 else
-$(error Could not find docker or podman in path!)
+$(warning Could not find docker or podman in path! This may result in targets requiring a container runtime failing!) 
 endif
 
 KUSTOMIZE_BUILD_DIR := config/default