It is true that the technologies existed for a while. I played with (Free)BSD jails and Solaris Zones long before Docker. Docker however made things "trivial" with the integration from container building, to public registry and docker-compose, which can make developer lives (depending on domain) much nicer. With jails and zones it was never as convenient.
That was docker's real value add, making it easy and developer centric. Previous containerization technologies were heavily ops-centric. I come from an ops background and I remember thinking at the time that Docker was in some ways a developer workaround to barriers that ops set up to limit what they could do and protect them from themselves.
