README on DockerHub does not get updated automatically

[saraedum]

Since Docker Hub made hooks/ a premium feature, our README does not get updated automatically from the one in our repository anymore.

Unfortunately, we can only have very few accounts on Docker Hub that have access to the sagemath organization. (More would also be a premium feature.)

So, currently the README can only be updated manually by the few who have access there.

CC: @slel

Component: docker

Issue created by migration from https://trac.sagemath.org/ticket/33408