Use Docker Desktop proxy when available

[gtardif]

• Check if Desktop is running and uses Desktop proxy for all download operations if yes
• timeout checking if desktop is running
• memoize it

[docker-agent]

Assessment: 🔴 CRITICAL

This PR introduces Docker Desktop proxy support but has several implementation issues that need to be addressed.

[docker-agent]

Assessment: 🔴 CRITICAL

Found critical bugs in the Docker Desktop proxy integration:

🔴 Critical: Missing ProxySocket for WSL2 and Native Linux

File: pkg/desktop/sockets_linux.go

The PR adds ProxySocket to DockerDesktopPaths and updates the LinuxKit case (line 15 in diff), but forgets to update the WSL2 case (line 26 in file) and native Linux case (line 40 in file).

Current code (WSL2):

return DockerDesktopPaths{
    BackendSocket: "/mnt/wsl/docker-desktop/shared-sockets/host-services/backend.sock",
    // ProxySocket is missing!
}, nil

Current code (native Linux):

return DockerDesktopPaths{
    BackendSocket: filepath.Join(home, ".docker", "desktop", "backend.sock"),
    // ProxySocket is missing!
}, nil

Impact: When NewTransport() calls desktop.Paths().ProxySocket, it will get an empty string on WSL2 and native Linux, causing socket.DialUnix() to fail. All HTTP operations will break on these platforms.

Fix: Add ProxySocket to both cases:

// WSL2 case:
return DockerDesktopPaths{
    BackendSocket: "/mnt/wsl/docker-desktop/shared-sockets/host-services/backend.sock",
    ProxySocket:   "/mnt/wsl/docker-desktop/shared-sockets/host-services/httpproxy.sock",
}, nil

// Native Linux case:
return DockerDesktopPaths{
    BackendSocket: filepath.Join(home, ".docker", "desktop", "backend.sock"),
    ProxySocket:   filepath.Join(home, ".docker", "desktop", "httpproxy.sock"),
}, nil

⚠️ Medium: Context handling in memoizer

File: pkg/remote/transport.go line 26

The memoizer caches IsDockerDesktopRunning(ctx) for 1 minute. If the first call's context is cancelled during the check, an incorrect result (false negative) gets cached.

Recommendation: Use a shorter cache duration or validate context state before using cached results.

⚠️ Medium: Context cancellation in IsDockerDesktopRunning

File: pkg/desktop/running.go line 9

The function creates a new 3-second timeout context derived from the passed context. This is actually correct - context.WithTimeout(ctx, ...) properly propagates cancellation from the parent context. However, the memoization means this check only runs once per minute, so context cancellation won't be respected for cached results.

---

Summary: The critical issue is that ProxySocket is missing for WSL2 and native Linux. This will cause the proxy integration to fail on these platforms. Please add ProxySocket to all return statements in getDockerDesktopPaths().

[docker-agent]

Assessment: 🔴 CRITICAL

Found 2 HIGH and 3 MEDIUM severity issues in the Desktop proxy implementation.

Critical Issues:

• Missing ProxySocket configuration for WSL2 and native Linux environments will cause proxy routing to fail completely on these platforms

Medium Issues:

• Context handling issues in memoization and HTTP requests