Customize the Per-User Postgres Docker Image

We provide each student on data100 witha postgresql server. We want the python extension installed. So we inherit from the upstream postgresql docker image, and add the appropriate package.

This image is in images/postgres. If you update it, you need to rebuild and push it.

1. Modify the image in images/postgres and make a git commit.
2. Run chartpress --push. This will build and push the image, but not put anything in YAML. There is no place we can put thi in values.yaml, since this is only used for data100.
3. Notice the image name + tag from the chartpress --push command, and put it in the appropriate place (under extraContainers) in data100/config/common.yaml.
4. Make a commit with the new tag in data100/config/common.yaml.
5. Proceed to deploy as normal.