BuildKit with custom builder does not route to localhost with IPv6

[vnmabus]

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• I've read the contributing guidelines and wholeheartedly agree. I've also read the issue reporting guide.

Well-formed report checklist

• I have found a bug that the documentation does not mention anything about my problem
• I have found a bug that there are no open or closed issues that are related to my problem
• I have provided version/information about my environment and done my best to provide a reproducer

Description of bug

Bug description

When running BuildKit with a custom builder, it is not possible to connect to localhost using IPv6 (connections time out).

Reproduction

Create a new builder:

docker buildx create --use --name kk

Create a new build (using docker buildx build). Run the following command as part of the build (or in a terminal inside the build container with Docker Desktop):

nc -v -w 5 localhost 14356

The result is:

nc: localhost ([::1]:14356): Operation timed out

However, in a normal Docker container, or when not using a custom builder the result is instead:

nc: connect to localhost port 14356 (tcp) failed: Connection refused

The same behavior can be tested with other network commands. For example, it is not possible to ping localhost IPv6 address.

The problem was detected when trying to compile Golang and run its tests, as one test tries to connect to a non-existing localhost port, and it fails with timeout instead of failing immediately, thus failing the test suite.

Note: I am using the Docker engine in Docker Desktop. I am not sure if that is relevant.

Version information

> docker buildx version
github.com/docker/buildx v0.19.2-desktop.1 412cbb151f1be3f8a94dc4eb03cd1b67f261dec5

> docker buildx inspect
Name:          kk
Driver:        docker-container
Last Activity: 2025-03-04 17:53:18 +0000 UTC

Nodes:
Name:                  kk0
Endpoint:              desktop-linux
Status:                inactive
BuildKit daemon flags: --allow-insecure-entitlement=network.host

> docker version
Client: Docker Engine - Community
 Version:           27.5.1
 API version:       1.47
 Go version:        go1.22.11
 Git commit:        9f9e405
 Built:             Wed Jan 22 13:41:31 2025
 OS/Arch:           linux/amd64
 Context:           desktop-linux

Server: Docker Desktop 4.37.1 (178610)
 Engine:
  Version:          27.4.0
  API version:      1.47 (minimum version 1.24)
  Go version:       go1.22.10
  Git commit:       92a8393
  Built:            Sat Dec  7 10:38:57 2024
  OS/Arch:          linux/amd64
  Experimental:     false
 containerd:
  Version:          1.7.21
  GitCommit:        472731909fa34bd7bc9c087e4c27943f9835f111
 runc:
  Version:          1.1.13
  GitCommit:        v1.1.13-0-g58aa920
 docker-init:
  Version:          0.19.0
  GitCommit:        de40ad0

> docker info
Client: Docker Engine - Community
 Version:    27.5.1
 Context:    desktop-linux
 Debug Mode: false
 Plugins:
  ai: Ask Gordon - Docker Agent (Docker Inc.)
    Version:  v0.5.1
    Path:     /usr/lib/docker/cli-plugins/docker-ai
  buildx: Docker Buildx (Docker Inc.)
    Version:  v0.19.2-desktop.1
    Path:     /usr/lib/docker/cli-plugins/docker-buildx
  compose: Docker Compose (Docker Inc.)
    Version:  v2.31.0-desktop.2
    Path:     /usr/lib/docker/cli-plugins/docker-compose
  debug: Get a shell into any image or container (Docker Inc.)
    Version:  0.0.37
    Path:     /usr/lib/docker/cli-plugins/docker-debug
  desktop: Docker Desktop commands (Beta) (Docker Inc.)
    Version:  v0.1.0
    Path:     /usr/lib/docker/cli-plugins/docker-desktop
  dev: Docker Dev Environments (Docker Inc.)
    Version:  v0.1.2
    Path:     /usr/lib/docker/cli-plugins/docker-dev
  extension: Manages Docker extensions (Docker Inc.)
    Version:  v0.2.27
    Path:     /usr/lib/docker/cli-plugins/docker-extension
  feedback: Provide feedback, right in your terminal! (Docker Inc.)
    Version:  v1.0.5
    Path:     /usr/lib/docker/cli-plugins/docker-feedback
  init: Creates Docker-related starter files for your project (Docker Inc.)
    Version:  v1.4.0
    Path:     /usr/lib/docker/cli-plugins/docker-init
  sbom: View the packaged-based Software Bill Of Materials (SBOM) for an image (Anchore Inc.)
    Version:  0.6.0
    Path:     /usr/lib/docker/cli-plugins/docker-sbom
  scout: Docker Scout (Docker Inc.)
    Version:  v1.15.1
    Path:     /usr/lib/docker/cli-plugins/docker-scout

Server:
 Containers: 20
  Running: 2
  Paused: 0
  Stopped: 18
 Images: 3
 Server Version: 27.4.0
 Storage Driver: overlayfs
  driver-type: io.containerd.snapshotter.v1
 Logging Driver: json-file
 Cgroup Driver: cgroupfs
 Cgroup Version: 2
 Plugins:
  Volume: local
  Network: bridge host ipvlan macvlan null overlay
  Log: awslogs fluentd gcplogs gelf journald json-file local splunk syslog
 CDI spec directories:
  /etc/cdi
  /var/run/cdi
 Swarm: inactive
 Runtimes: io.containerd.runc.v2 runc
 Default Runtime: runc
 Init Binary: docker-init
 containerd version: 472731909fa34bd7bc9c087e4c27943f9835f111
 runc version: v1.1.13-0-g58aa920
 init version: de40ad0
 Security Options:
  seccomp
   Profile: unconfined
  cgroupns
 Kernel Version: 6.10.14-linuxkit
 Operating System: Docker Desktop
 OSType: linux
 Architecture: x86_64
 CPUs: 8
 Total Memory: 7.519GiB
 Name: docker-desktop
 ID: 0c9854ee-a6d4-4cf8-b65d-72c9aaadcb0b
 Docker Root Dir: /var/lib/docker
 Debug Mode: false
 HTTP Proxy: http.docker.internal:3128
 HTTPS Proxy: http.docker.internal:3128
 No Proxy: hubproxy.docker.internal
 Labels:
  com.docker.desktop.address=unix:///home/user/.docker/desktop/docker-cli.sock
 Experimental: false
 Insecure Registries:
  hubproxy.docker.internal:5555
  127.0.0.0/8
 Live Restore Enabled: false

WARNING: daemon is not using the default seccomp profile