[docker] Build with 3.12 image

[cooperlees]

• We are a 3.12 package now, let's build docker with it

Test:

crl-m1:black cooper$ docker build --tag black_3_12 .
...
 => [stage-1 2/2] COPY --from=builder /opt/venv /opt/venv                                                                                                                                                  0.2s
 => exporting to image                                                                                                                                                                                     0.1s
 => => exporting layers                                                                                                                                                                                    0.1s
 => => writing image sha256:bd66acc9d76d2c40d287b0684ce6601401631e0468204c4e6a81f8f1eebaf1dd                                                                                                               0.0s
 => => naming to docker.io/library/black_3_12

crl-m1:black cooper$ docker image ls | grep black_3_12
black_3_12                     latest            bd66acc9d76d   59 seconds ago   193MB

Fixes #3919