Question: So I have attempted to dive into Docker and created my first image of a web app and can run it successfully.

How can I create a .deb file to send this out to people to install it on their systems?

Answer: 
Docker images themselves are what should be distributed. Images are not meant to be converted to .deb packages. You should look into publishing the image you created on [docker hub](https://docs.docker.com/docker-hub/) or another container repository like [Amazon Elastic Container Registry](https://aws.amazon.com/ecr/) or [GitHub Container Registry](https://docs.github.com/en/packages/working-with-a-github-packages-registry/working-with-the-container-registry) so people can `docker pull` and `docker run` your image.

If you don't want to use a container registry, you can always share your repository and `Dockerfile` so people can build it locally.

See the [other answer](https://stackoverflow.com/a/68253442/4676641) for a more reliable way but not "best practice" method to share Docker images. These methods all require Docker be installed.
Another way to send an image to someone - without using a registry - is to use `docker save` to create a file containing the image. You can then send the file to the people that need it and they can restore the image using `docker load`.