You can just run an interactive shell container using that image and explore whatever content that image has.
For instance:
docker run -it image_name sh
Or, if you want to see how the image was build, meaning the steps in its Dockerfile, you can:
docker image history --no-trunc image_name > image_history
The steps will be logged into the image_history file.
For images with an entrypoint configured, you can run docker run -it --entrypoint sh image_name
instead to override it (helpfully suggested in the comments).