refactor: Add build target to Makefile for PostgreSQL image creation

Author: ruxwez

Makefile

@@ -1,5 +1,10 @@
-.PHONY: test
+PG_VERSION ?= 18
+
+.PHONY: test build
 
 test:
 	docker build -f .docker/Dockerfile.test -t postgres-extensor-test .
 	-docker rmi postgres-extensor-test > /dev/null
+
+build:
+	docker build -f .docker/Dockerfile -t ruxwez/postgres:$(PG_VERSION) --build-arg POSTGRES_VERSION=$(PG_VERSION) .