This "Task" is a collection of ideas for improving the CI infrastructure. The overall goal is to improve our code quality.

• Replace Travis with our own CI system for running CI tasks. Currently, we're using Scrutinizer for running code quality checks, additionally to the checks we run on Travis, which seems wasteful. Also, it could enable us to keep more detailed metrics on code quality together with historical data (code quality metrics over time).
• Use SonarQube to collect and visualize metrics.
• Add architectural checks (layer violation) by checking dependencies with deptrac or dephpend

See also T162457: Use containers for testing feature branches - when that task is done, our CI system could deploy the current branches to the test system