This is a common misconception but there's plenty of info out there if you want to find out how to do it. https://duckduckgo.com/?q=reproducible+builds+in+docker&atb=...
In essence, you can make reproducible builds in docker by controlling the build environment and then specifying particlar versions when installing dependencies. It's a bit more work but it's eminently doable.
You control the build environment by building your reproducible docker containers within an environment (eg a reproducible docker container) that is itself reproducible (ie with specified versions of everything). This seems like a bootstrapping problem, but actually it works.
So: 1. Make a "Build container" with a specified base image version and specified versions of the transitive deps of everything it needs to build your thing 2. Use this container to build your actual container, specifiying a version of the base image and all dependencies.
Pretty sure you don't need more than these two layers of the onion to make bitwise-identical builds as long as your package build recipe is itself reproducible (eg the toolchain supports it and you don't do anything like embedding timestamps etc in the binaries).
Package managers provide mechanisms to get the version list in force so generally the way to do this is to start with your preferred base image, install all the packages you want/need, use the package manager to give you the version list, then change the Dockerfile to install specifically those versions.