Implement Docker support in integration tests

[notgull]

Creates a Docker image with xvfb in it and adds an option to use it to run integration tests.

Right now it seems that the X11 backend does not support XVFB, which requires further investigation. So I haven't added it to CI just yet.

• [x] Tested on all platforms changed
• [x] Added an entry to CHANGELOG.md if knowledge of this change could be valuable to users
• [x] Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• [x] Created or updated an example program if it would help users understand this functionality
• [x] Updated feature matrix, if new features were added or implemented