Okay, so everybody uses Jenkins. I don't like it. It doesn't instill comfort in me when I see how much functionality it lacks out of the box, how flakey crucial plugin updates are, and the pain and learning curve people endure writing Groovy.
GitLab CI is wonderfully simple. If you're already a GitLab VCS user, then GitLab CI does exactly what you want 9 times out of 10.
I'm taking a look at GoCD and Concourse CI as alternatives to Jenkins. Lets find out what we're like, and how well they cater for Dockerized pipelines.... ;-)