[yalooze]

I think they're saying they run the pipeline builds using docker containers. It's how they can build easily based on any branch. Ie. what they use behind-the-scenes, not what you need to be using. Although I'm somewhat doubting myself now...

[ibuchanan]

That's correct. Bitbucket Pipelines uses Docker as an execution environment for builds. However, if the goal of your build is to produce a container, whether Docker or otherwise, Bitbucket Pipelines cannot do that at this time.