Docker does not give you a reproducible builds - it 100% depends on a host platform.
Also, any Dockerfile that downloads something that might be updated (especially package managers, but also any wget/curl/... that doesn't check against a hardcoded hash) isn't reproducible at all.
Recently somebody showed me their sophisticated Docker file that was supposed to build a Maven project "in a reproducible way". When you ran it it spent 5 minutes downloading a bazillion files from the internet…no prior information on the content of those files was on the system. If the content of version X of a dependency changed in the meantime, you'd never know.
I admit some people definitely abuse the usage of Docker, because of their bad copy/paste practices from basic step-by-step walk-throughs. But the thing that makes docker great is it's easy of use to create reproducible deployments. And if you don't want to use a projects docker images; a projects Dockerfile is still a great reference on how to get a project deployed on a linux platform.