Overriding the default command

Overriding the start command of an image has many uses. One of them is testing or debugging something in an image that already has a start command with a specific use case. For example, the postgres image starts a database if you start it normally. To be able to dive into the image and look at configuration files or debug an issue you're having; you can start it with the bash command, an often available shell. What is the command to start the postgres image interactively with a bash shell?