Fix linux docker proxy

[matejkramny]

Fixes #3239

I added some logging to the proxy and observed the /attach API call converting to a websocket. This websocket is streaming data to/from the container.
I suspected the issue stemming from nonstandard docker behaviour when it comes to websockets, as one of the pipes gets closed when the container does not have stdin open.

The docker client knows the container spec and when stdin on the container is closed, outbound pipe from the client is EOF whilst the inbound pipe (stdout/err from container) is streaming data.

The issue can be worked around if you set DOCKER_HOST to /mnt/wsl/rancher-desktop/run/docker.sock effectively isolating the issue to the wsl-helper proxy.

The PR is proof of concept - starts a proxy on the TCP level, and conditionally sends connections to the original proxy capable of munging the request data etc.

To verify fix, run

go build -o ./wsl-helper ./ && sudo ./wsl-helper docker-proxy serve --verbose --endpoint unix:///tmp/docker-wsl.sock --proxy-endpoint /mnt/wsl/rancher-desktop/run/docker.sock

export DOCKER_HOST=unix:///tmp/docker-wsl.sock
docker run --rm -p 8888:80 nginx

used https://github.com/jpillora/go-tcp-proxy
and https://github.com/docker/go-connections/blob/master/sockets/inmem_socket.go

[matejkramny]

screenshot shows docker cli closing one of the streams when stdin isn't enabled on the container

https://github.com/docker/cli/blob/master/cli/command/container/hijack.go#L132
from https://github.com/docker/cli/blob/master/cli/command/container/attach.go#L150

[matejkramny]

Go's httputil.ReverseProxy is not capable of proxying "half closed" tcp connections, ref linked issues starting here:

golang/go#70754

We can bypass the http proxying by going down to L4 and starting a tcp proxy between the two sockets and conditionally forward the connection to L7 when the request matches the criteria.

[matejkramny]

Possibly fixes #2094

@jandubois would you be able to review?

Thank you

[MrWako]

Great analysis @matejkramny - think it might be worth trying to get back a fix to the Go reverse proxy in the first instance, something like: golang/go#35892.