Lots of Docker fans don't think about what version of libc or java is in their image. They start with a base image, develop code that works there, and release the docker container without ever thinking about it.
If it's an open source project, ignoring bugs that occur outside the official docker build cuts out a lot of work. Inevitably someone with no Linux experience will try to setup Slackware on a raspberry pi to run your project because they read in a forum post that 'real nerds run slackware.' When it doesn't work and they open a bug report, you can spend the next year trying to teach them enough about Linux for them to fix their system and run your project. Or you can come across as mean by saying RTFM. Or you can just avoid all that by pointing them to the Docker image and admitting "I only have the resources to ensure this works in that specific environment."
If it's a corporate project, it safeguards against other devs or someone in IT being foolish. Despite having rules and procedures in place, I've seen plenty of instances where IT or a dev changed something in production environments without warning or announcing it because they thought it would be fine.