Docker is not repeatable. If I try to build the same Dockerfile in 12 months then I likely do not get the same output and it wouldn't be surprising if it does not build at all.
The notion of portability is Docker's bread and butter. With proper version pinning you _might_ get lucky with repeatability too. Most people anticipate repeatability to be part of portability, but it really is a distinct notion.
> it wouldn't be surprising if it does not build at all.
This part is a stronger point, and it's true. When you repeat something and what you get is not just a different version or variation but something with totally different or broken behavior, that seems bigger than an ordinary failure of reproducibility.