build: sublogger to show docker load progress output

[crazy-max]

Adds sublogger to show load progress when importing to docker:

record-2023-08-10.105831.mp4

$ docker buildx create --name foo
$ docker buildx --builder foo bake --load https://github.com/crazy-max/docker-docker.git
...
#35 exporting to docker image format
#35 exporting layers
#35 exporting layers 7.3s done
#35 exporting manifest sha256:3ddc93cb4082a582552e7b32cfe439ae4fc2c003f353a8259e055a8b9e373fb6 0.0s done
#35 exporting config sha256:b2f1eaf0595fd4ccb26c63715aca3698d6433d9e34220cd8db358e0332d2b18e 0.0s done
#35 sending tarball
#35 ...

#36 importing to docker
#36 loading layer 4270763b4b7f 294.91kB / 27.73MB 3.0s done
#36 loading layer fc8ab555be16 557.06kB / 70.77MB 2.5s done
#36 loading layer b7eb284ce929 163.84kB / 13.78MB 1.7s done
#36 loading layer 13a4dd8d8fd7 557.06kB / 52.93MB 1.5s done
#36 loading layer 5f70bf18a086 32B / 32B 1.0s done
#36 loading layer a7f29fd70512 196.61kB / 17.46MB 1.0s done
#36 loading layer a950fc94a643 196.61kB / 17.78MB 0.8s done
#36 loading layer d859045f8607 458.75kB / 45.74MB 0.5s done
#36 loading layer 12aba25a1c9f 332B / 332B 0.1s done
#36 loading layer 0dbb41e3c807 116B / 116B 0.1s done
#36 loading layer 089d02ae3a66 546B / 546B 0.1s done
#36 loading layer a08aa04b3edb 1.02kB / 1.02kB 0.1s done
#36 DONE 3.0s

[jedevc]

Not a blocker: I wonder if we should show layer extraction? Especially on lower-powered machines (e.g. raspberry pis), this is probably quite a slow step.

I put together an idea of how we might do something like this in moby/buildkit#4041 - the issue is that it involves updating the Name of a VertexStatus which today isn't supported (or we end up having to have 2 VertexStatuss, one for downloading, one for extracting, which clogs up the progress output).

[tonistiigi]

1 or 2 is fine in here. 5sec already feels like something might be stuck if there are not many layers.