feat: add support for docker-socket-proxy

[dan-hollis]

Summary

Adds TCP Docker connection support via DOCKER_HOST for use with docker-socket-proxy and similar TCP-based proxies.

Previously, DOCKER_HOST was loaded from the environment but never actually wired into the Docker client. All connections were hardcoded to Unix sockets. This caused startup failures when attempting to use a TCP proxy:

• Docker socket not found at tcp://socket-proxy:2375 (config validation ran os.Stat() on a TCP URL)
• dial unix /var/run/docker.sock: connect: no such file or directory (client always dialed Unix)

Changes

• internal/docker/client.go: New NewClientWithHost() constructor with TCP/Unix transport selection. New DialDocker() method as single entry point for all raw connections, replacing 5 scattered net.Dial("unix", ...) calls.
• internal/docker/compose.go: New dockerHostEnv() helper so subprocesses (docker compose, docker login) receive the correct DOCKER_HOST value instead of hardcoded unix://.
• internal/config/config.go: Validates DOCKER_HOST format (tcp:// required) when set; skips socket file check.
• internal/server/http.go, internal/edge/client.go, internal/edge/tunnel.go: Updated to pass DockerHost through and use DialDocker() for raw connections.

Usage

environment:
  - DOCKER_HOST=tcp://socket-proxy:2375

-e DOCKER_HOST=tcp://socket-proxy:2375

Where socket-proxy is a docker-socket-proxy container on the same internal docker network in this case.

fixes #44
fixes #46