cli: Add build-if-not-exists command

This patch allows to optionally build the koolbox container on install
if it isn't already present on the system. This should make the usage as
part of the uplevel CLI easier.

cli/Makefile

@@ -14,13 +14,17 @@ preflight:
 	command -v podman > /dev/null
 
 .PHONY: install
-install: preflight ## Install koolbox, the Kubernetes Toolbox for SI-Infrastructure
+install: preflight build-if-not-exists ## Install koolbox, the Kubernetes Toolbox for SI-Infrastructure
 	install -m 755 ./koolbox ~/bin/koolbox
 
 .PHONY: build
 build: preflight ## Build container locally instead of using one from upstream
 	podman build -t $(KOOLBOX_IMAGE) .
 
+.PHONY: build-if-not-exists
+build-if-not-exists: preflight
+	podman image exists $(KOOLBOX_IMAGE) || make build
+
 .PHONY: config
 config: ## Edit your koolbox environment config
 	${EDITOR} "${XDG_CONFIG_HOME}/koolbox/env"