Proxy command doesn't handle websockets

[Juan Carlos Mejías Rodríguez]

Describe the bug
When using PSU as a proxy to an endpoint's Docker daemon, commands requiring a websocket connection (like docker attach, docker exec, docker system events) fail with an unable to upgrade to tcp, received 200 error or just hang up.

To Reproduce
Given:

• There is a "primary" endpoint
• There is a "backend" container running on that endpoint
• There is a Docker client installed locally

Steps to reproduce the behavior:

1. Run psu proxy --endpoint primary --port 12345
2. In another shell, run export DOCKER_HOST=tcp://localhost:12345
3. Run docker exec backend echo "hello"
4. See error

Expected behavior
PSU should correctly proxy the request by handling the protocol upgrade

Standard/Error output

unable to upgrade to tcp, received 200

Standalone executable binary (please complete the following information):

• OS: Linux
• Shell interpreter: Bash
• Portainer version: 1.21.0
• Program version: 2.0.0-alpha.3