Based upon the official Jenkins image, this simply adds the scm-api and git-client plugins.
The base image specifies that you can create a plugins.txt and use that, but the Docker Hub's automated build didn't appear to be downloading all the plugins.
So the answer is to ADD the plugins directly to /usr/share/jenkins/ref/plugins directory, making sure they've got the right permissions for Jenkins.
To help you with this, the build_dockerfile.sh file creates a Dockerfile based upon plugins.txt, ADDing each additional plugin manually.
Edit plugins.txt
# Comments are ignored, as are empty lines
#<plugin_name>:<version>
ansicolor:latest
Now create the Dockerfile
./build_dockerfile.sh
docker pull andrewgortonuk/dockerjenkinswithgit:latest
docker run -p 8080:8080 andrewgortonuk/dockerjenkinswithgit:latest