fix: route OCI artifact pulls through Docker Desktop HTTP proxy

The compose process performs OCI artifact fetches in-process via
containerd's docker resolver, whose default transport only honors
HTTP_PROXY/HTTPS_PROXY/NO_PROXY env vars. Users behind PAC-only
corporate proxies hit i/o timeouts on `oci://` includes and on
`compose publish`.

When Docker Desktop is the active engine and exposes httpproxy.sock,
route the resolver through it (PAC-aware). Falls back to the default
transport when DD is unavailable or the socket is missing. Modeled on
docker/mcp-gateway PR #354.

Signed-off-by: Guillaume Lours <glours@users.noreply.github.com>