Because in GCC's confusing versioning scheme [1], "GCC 9.0" was never a stable release; that version number was used exclusively for development builds. So GCC 9.1 is the first GCC 9 worth packaging, and it just came out. (And by the time of my comment, the Docker page you linked has already been updated to mention it.)
[1] https://gcc.gnu.org/develop.html