503 with docker-desktop on Mac

With docker-desktop 4.20.1 on Mac, the following crashes with a HTTP 503 error:

# get the location of the currently active docker socket, will be something like $HOME/.docker/run/docker.sock
#
socket="$(docker context ls --format '{{.Current}}\t{{.DockerEndpoint}}' | awk '$1 == "true" { print $2 }')"
socket="${socket#unix://}"

# run the socket proxy
#
docker run -p 2375:2375 -v "${socket}:/var/run/docker.sock:ro" -d --rm tecnativa/docker-socket-proxy

# make a call to the proxy: this crashes
#
DOCKER_HOST=tcp://127.0.0.1:2375 docker ps

Hard-coding socket=/var/run/docker.sock works fine, but shouldn't there be a way to respect the socket location provided by the docker context?

Other approaches I tried which didn't work:

• Adding --privileged to the docker run command: same crash with a 503
• Adding --user $UID:$GID to the docker run command: container fails to start with error "Cannot create pidfile /run/haproxy.pid"

Start by reproducing the failure with Docker Desktop 4.20.1 using the docker context-derived socket, docker-socket-proxy, and DOCKER_HOST=tcp://127.0.0.1:2375. Compare it with the working hard-coded /var/run/docker.sock setup and trace the source of the HTTP 503; done means the context-provided socket works through the proxy without the crash.