Since May last year we have an alpine-based image of juanluisbaptiste/postfix being tested to replace the current one based on CentOS 7. The switch of base image was needed because CentOS 8 does not include supervisord, which is the process control system we use inside the container. We were not going to be able to continue using CentOS as base image unless another process control system was used.
So I decided to use this occasion to switch to Alpine linux as the base image. This has another big advantage with the resulting image size being drastically reduced (from 393MB to 135MB uncompressed), making the SMTP relay more lightweight. After months of testing in production and a real lack of time to finish this I was able to merge the alpine branch into master. Now the alpine image is the default one. This means both the latest and alpine tags now points to the alpine image. The old CentOS image can be found at the old_centos_base branch if needed.
If you have any issues please report them in the issue tracker.