fix(publish): bypass Docker Desktop proxy for localhost/insecure registries

[ptrdom]

What I did

docker compose publish routed all registry traffic through Docker Desktop's HTTP proxy. Publishing to a localhost/insecure registry therefore failed on Windows with:

proxyconnect tcp: open ./pipe/dockerHttpProxy: The system cannot find the path specified.

even though docker push/docker pull worked against the same registry.

Two bugs in internal/desktop/proxy.go:

1. No loopback bypass. ProxyTransport forced every request through the DD proxy and its DialContext always dialed the proxy socket, so loopback targets could never connect directly. Proxy selection now goes through httpproxy.Config.ProxyFunc, which exempts localhost/loopback and additionally honors NO_PROXY. DialContext routes only the sentinel proxy address to the DD socket and dials real targets directly otherwise.
2. Malformed Windows pipe path. The proxy named-pipe endpoint was hardcoded as npipe://./pipe/..., yielding the relative path ./pipe/dockerHttpProxy. It is now derived from the engine endpoint, preserving its namespace. Docker Desktop reports the endpoint in the backslash form npipe://\\.\pipe\docker_cli, so the derivation uses LastIndexAny to handle both backslash and forward-slash forms.

Result: publishing to localhost:5000 connects directly like docker push, while the DD proxy still serves external registries (now reachable on Windows).

Related issue

Fixes #13824

How I tested

Validated end-to-end on Docker Desktop 29.5.2 / Windows 11 (the issue's environment) by building binaries from both the parent commit and this branch and running the issue's reproduction steps:

# start a plain-HTTP localhost registry
docker run -d -p 5000:5000 --name registry registry:3

# minimal compose.yaml referencing any pushable image, then publish
# (spike-app is the placeholder image name from #13824's repro steps)
docker compose publish localhost:5000/spike-app:1 --insecure-registry -y

• Parent commit: reproduces the issue verbatim — proxyconnect tcp: open ./pipe/dockerHttpProxy: The system cannot find the path specified. (exit 1).
• This branch: publishes successfully (exit 0); the artifact lands in the localhost registry.

Automated coverage in internal/desktop/proxy_test.go:

• TestDDProxyFunc_BypassesLoopbackAndHonorsNoProxy — loopback names/IPs and NO_PROXY hosts bypass; external hosts proxy.
• TestHTTPProxySocketEndpoint_WindowsNamedPipe — proxy pipe path keeps the engine endpoint's namespace for both backslash and forward-slash forms.

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🤖 Generated with Claude Code

[ptrdom]

I have also manually tested this on MacOS (M1 Mac) with Docker Desktop, managed to reproduce the issue and confirmed that this PR fixes it.