You can see an example of a step that took a long time (18 minutes) here: https://github.com/teohhanhui/api-platform-core/runs/209206317#step:10:55
In our existing build, running the same step on Travis CI is much faster (less than 5 minutes): https://travis-ci.org/api-platform/core/jobs/579269270#L514
If there are any pecularities when using a Docker container, it should be documented. For example, how is the workspace being made available to the Docker container? Is it a bind-mounted directory from host? What is the Docker storage driver being used?