Docs: Document building custom Dependency Scanning image

Problem

It's not uncommon for customers to want to customize the Dependency Scanning Docker image in order to deviate from how we've set it up e.g. they may wish to use an alternative Java distribution such as Amazon Corretto.

Solution

Using this merge request as a basis, write some documentation to capture what's involved in creating a custom image. A starting point may be to write this as development documentation in the Gemnasium project.

/cc @thiagocsf