You need the docker-engine and Kitematic installed on your local machine. For macOS have a look here for docker and a download link for Kitematic will appear in the menu of the docker status icon.
Find docker images
Just start Kitematic and search for magnolia. You'll find a handful of repositories and the ones to look for are all part of just magnolia. So for example magnolia/community-demo. https://hub.docker.com/r/magnolia/community-demo/
By default you will get the latest version but you can select a specific version if you select the tag in the ellipses menu:
latest btw means the latest version from the master branch.
The version or tag is combined out of the version of magnolia (e.g. the specified version in the webapp project pom) and the build number of the jenkins job this image was created in.
Start magnolia docker container
After you found the right image click on Create to get a magnolia docker container running. After magnolia has finished starting & installing (it should stop producing log entries end with INFO: Server startup in .... ms) you can click on Web Preview. It will open an empty browser but with the correct port and you have to manually add either magnoliaAuthor or magnoliaPublic and everything should work as expected.
Multiple containers and ports
You can start multiple magnolia containers this way. They will be all available under different ports which are dynamically created by docker. If you click on Web Preview the newly opened browser will already have the correct port so you don't have to care about that "detail". But if you really want to see the details look under Settings->Hostname/Ports
Use light modules
Click on the Volumes on the right side and Kitematic will map the volumes to a local folder so you can easily access the files or create light module files.