First thing to understand about Mariner is that is not a general purpose Linux distribution like Ubuntu or Fedora, it was created by Microsoft’s Linux System Group which is the same team at Microsoft which created the Linux kernel used for Windows Subsystem for Linux version 2, or WSL2. The goal of Mariner is to be used as an internal Linux distribution for Microsoft’s engineering teams to build our cloud infrastructure and edge products and services.
Of course Mariner is open source and it has its own repo under Microsoft’s GirHub organization. No ISOs or images of Mariner are provided, however the repo has instructions to build them on Ubuntu 18.04. There are a series of prerrequistes listed in this GitHub page that roughly include Docker, RPM tools, ISO build tools and Golang, amongst others.
Not surprising, of course, but still quite interesting to poke around in.