runControllerBuild: Read from stdin only when needed

[ktock]

Current forwarder implementation aggressively reads from the reader registered via SetReader. And in runControllerBuild, stdin is always registered to the forwarder by default. But there are cases where stdin should not be read so this commit fix this behaviour to register the stdin to the forwarder only when actual read happens.

Related to docker/cli#4792

[ktock]

Maybe this is related to docker/cli#4792 ?

cc @crazy-max @jedevc help wanted for testing 🙏

[crazy-max]

LGTM thanks @ktock!

Tested on my side with cli 25.0.0-rc.2

BUILDX_EXPERIMENTAL=1 ./build/docker-linux-amd64 version
Client:
 Version:           25.0.0-rc.2.m
 API version:       1.44
 Go version:        go1.21.6
 Git commit:        1fc6ef9d63
 Built:             Tue Jan 16 16:19:36 2024
 OS/Arch:           linux/amd64
 Context:           default

Server: Docker Desktop
 Engine:
  Version:          25.0.0-rc.1
  API version:      1.44 (minimum version 1.24)
  Go version:       go1.21.5
  Git commit:       9cebefa
  Built:            Thu Jan  4 16:30:11 2024
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.6.26
  GitCommit:        3dd1e886e55dd695541fdcd67420c2888645a495
 runc:
  Version:          1.1.10
  GitCommit:        v1.1.10-0-g18a0cb0
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

BUILDX_EXPERIMENTAL=1 ./build/docker-linux-amd64 buildx version
github.com/docker/buildx v0.12.0-rc2-77-g0dc15505 0dc1550573e45dbd0a374817cc0b310a9c6ec1d6

BUILDX_EXPERIMENTAL=1 ./build/docker-linux-amd64 build .
#0 building with "default" instance using docker driver

#1 [internal] connecting to local controller
#1 DONE 0.0s

#2 [internal] load build definition from Dockerfile
#2 transferring dockerfile: 4.90kB 0.1s done
#2 DONE 0.1s

#3 [auth] docker/dockerfile:pull token for registry-1.docker.io
#3 DONE 0.0s

#4 resolve image config for docker.io/docker/dockerfile:1
#4 DONE 0.8s

#5 docker-image://docker.io/docker/dockerfile:1@sha256:ac85f380a63b13dfcefa89046420e1781752bab202122f8f50032edf31be0021
#5 CACHED

#6 [internal] load metadata for docker.io/tonistiigi/xx:1.2.1
#6 ...

[ktock]

Thanks! Added small changes for atomicity.

So it seems that reading from stdin isn't allowed in some cases so stdin-aware operations (e.g. reading Dockerfile from stdin and launching (TTY-raw-mode) monitor REPL) won't be supported from docker cli in such cases?

[tonistiigi]

Is this still desired after docker/cli#4792 merge? I don't really see why we should have some protections against reading from stdin.