Once you are comfortable with launching containers and stacks you may wish to customise your images.
You can create a new image based on changes to an existing container by following the steps outlined in the Plesk Documentation
We recommend to use a meaningful tag name instead of latest (the default) because what is latest today will soon become outdated over time... You might use an incrementing version number or something containing the creation date as example alternatives.
Creating your own Docker image involves writing a Dockerfile, and then building it.
Docker images are composed in layers, meaning that it's normal for one image to inherit / be built upon another - so you don't need to start from scratch. However, be mindful that as well as inheriting someone else's work, you're inheriting their bugs and security vulnerabilities too!
Please refer to Docker's Building images guide for more details.