This isn't a footgun. A footgun is when something happens that should be expected, but isn't for reasons of negligence or ignorance to the thing that should be expected. The example that everyone seems to love is pointer arithmetic. If you make a basic error in your math, invalid memory access may occur and then likely more bad stuff.

Docker altering firewall rules without explicit instructions to do so is either a feature, if you're looking to defend docker, Or a security defect if you're feeling rational. There's an argument to be made for the root cause of the issue, The docker container manifest, or the application itself. But to pretend like this is what docker should be doing, users should know better just seems wrong.