I have two CI servers for testing against different database versions, and on each box I run three different versions of Postgresql, two versions of MySQL, two

I have two CI servers for testing against different database versions, and on each box I run three different versions of Postgresql, two versions of MySQL, two versions of MariaDB, Microsoft SQL Server for Linux, and Oracle express. They all spin up from dockerfiles and run in containers. Because they all see a clean linux host, I can use the standard RPM installers for all of them, they listen on their default ports, all the datafile layouts are out of the box from the package manager. Trying to recreate this system without using containers or VMs or something is much more difficult, because you can't for example install the MySQL 5.6 and MySQL 5.7 rpm packages on one host at the same time, you have to instead use a generic binary or build from source into custom /opt/ locations, as well as make sure everyone listens on a different port, uses unique config, etc. Oracle express gets extremely upset if things are not exactly the way it wants, and Oracle only installs from an rpm or deb package. The system used to work this way before and moving everything to docker just erased all the complexity and difficulty, it's now ridiculously easy to maintain, upgrade, and extend. Postgresql released the 10.x series some months ago, to add it alongside the 9.x versions all I had to do was add "10" to an ansible file and re-run the playbook to spin it up on the two CI workers. All of the code I use to build these servers is easily understandable and reusable by anyone familiar with Docker and Ansible. Besides that, I use docker containers for hosts where I have for example services running that need to use particular VPNs, using the containers so that each service has its own network environment. Again, I could instead get multiple OpenVPNs running all at once on the host and mess with the routing tables and hope that a new routing rule doesn't break one of the other services, but sticking everything in Docker containers again totally simplifies everything. I seem to be good at finding use cases where Docker makes things much simpler and a lot less work. I'm also using Ansible to orchestrate everything so I suppose I shouldn't get the author of this post started on that :).