Enable WCOW custom frontends to communicate with BuildKit via named pipe bridge

[billywr]

PR adds support for custom frontends in WCOW by enabling gRPC communication from the container to the BuildKit in host machine via a named pipe.

Currently, the custom frontend bridge used anonymous pipes, which work well for Linux containers but are not supported in WCOW environments. As a result, custom frontends running in Windows containers were unable to communicate with the host machine's BuildKit instance.

This change:

• Introduces a named pipe (\\.\pipe\buildkit-frontend-bridge) specifically for WCOW custom frontend containers.
• Mounts the pipe into the container .

Fixes #4892

Guide to test

STEP 1.
Compile frontend binary
You can use go code in this location: https://github.com/moby/buildkit/blob/master/frontend/dockerfile/cmd/dockerfile-frontend/main.go
use go build -o ... to create dockerfile-frontend.exe

STEP 2.
Build frontend image and push it your docker hub
The dockerfile contents to build the frontend image:

FROM mcr.microsoft.com/windows/nanoserver:ltsc2022
LABEL moby.buildkit.frontend.network.none="true"
LABEL moby.buildkit.frontend.caps="moby.buildkit.frontend.inputs,moby.buildkit.frontend.subrequests,moby.buildkit.frontend.contexts"

COPY dockerfile-frontend.exe C:\dockerfile-frontend.exe

ENTRYPOINT ["C:\\dockerfile-frontend.exe"]

Can use this buildctl command to build frontend image

buildctl build `
--frontend=dockerfile.v0 `
--local context="path to your build context" `
--local dockerfile="path to your dockefile" `
--output type=image,name=yourdockerhuburl/dockerfrontend:latest,push=true `
--no-cache 

STEP 3.
Use the frontend image in your docker builds

say you have a dockerfile below, notice the use of #syntax to make use of your custom dockerfile frontend

yourdockerhuburl/dockerfrontend:latest comes from the image you pushed in step 2

# syntax=yourdockerhuburl/dockerfrontend:latest

FROM mcr.microsoft.com/windows/nanoserver:ltsc2022
RUN echo hello from WCOW custom frontend

[iankingori]

creating a new uuid here may not be necessary, there should be a session Id that is unique to each build that we can leverage for this same purpose and would navigate the risk of the FrontendGRPCPipe env being overwritten