While security has nothing to do with my disgust for docker and people advocating its use, docker adds a layer of complexity, which means it is not necessarily more secure.
What is extremely bad about docker:
it enables extremely shitty configuration control on the side of a developer. There are way too many developers who have a chaotic approach to configurations, and instead of being forced to write a proper installation and configuration guide from scratch, and thereby making themselves(!) aware of active configuration changes they made to make their system work, they just roll out the docker container they develop in, without remembering most of the configurations they made. Which, naturally, means that they are unable to assist in troubleshooting problems or reproduce issues that users might have.
In general, if you can’t write a good user manual, or at least clearly identify needed dependencies and configurations, you should not be developing software for other people.
it combines the disadvantages of a VM (shitty performance) and running directly on the host OS (sandboxing is not nearly as good as on a VM)
it creates insane bloat, by completely bypassing the concept of shared libraries and making people download copies of software they already have on their system
it adds a lot of security risks because the user would have to not only review the source code they are compiling and installing, but also would have to scan all the dependencies and what-not, and would basically have to trust the developer and/or anyone distributing an image that they did not add any malware.