What are the base Ubuntu (16 and 18) images Pytorch and Tensorflow are based off of when built?

Essentially it’s all there in the question. Are they pulled from the official Ubuntu images found on dockerhub (https://hub.docker.com/_/ubuntu?tab=tags)?

There are many tags, and the ‘tag’ section of each pytorch / container does not indicate more than a hexidecimal reference to some container without additional information (that I can understand, anyways - perhaps the hexidecimals are a specific reference I should know how to interpret - If so let me know!).

If there’s dockerfiles to look at that’d be helpful too, but I’m more curious about just knowing what the base image is starting from.

Thanks,
DW