How can I troubleshoot Docker pull failing with HTTP 503 error? - docker-for-windows

Question

I’ve used docker some in the past , I’m trying to introduce it at a new environment, but cannot get docker pull working. Keeps failing with 503 response.
What I could use help with is finding the underlying cause of this issue, so I can have the net/it admins fix whatever it is, be it firewall, ssl, proxy authentication, (all the usual suspects running docker/windows in a corporate network, ya know.)
Where can I look to see the root cause of pull failures to help them out? Also any specific diagnostic tests I could run to help figure this out would help.
I am able to hit registry-1.docker.io in general:
e.g.
curl https://registry-1.docker.io/v2/
{“errors”:[{“code”:“UNAUTHORIZED”,“message”:“authentication required”,“detail”:null}]}
I’ve poked through the logs in %localappdata%/docker, but have not been able to find anything.
the only thing I found at all was in dockerd.log:
2021-08-19T13:59:06Z dockerd time=“2021-08-19T13:59:06.656666400Z” level=debug msg="pulling blob “sha256:9da81141e74e38839836e81c2691d3c7ac54bf34272e5d4a636fc032150506a4"”
2021-08-19T13:59:06Z dockerd time=“2021-08-19T13:59:06.994330700Z” level=info msg=“Download failed, retrying (1/5): received unexpected HTTP status: 503 Service Unavailable”
Which does not really help. I know docker registry is not down, so this is not really a 503 from docker.
System info
OS: Windows 10 19043, WSL2 enabled (VM with virtualization enabled, if that matters)
Docker version 20.10.7, build f0df350
repro steps:
docker run -d -p 80:80 docker/getting-started
Unable to find image ‘docker/getting-started:latest’ locally
latest: Pulling from docker/getting-started
540db60ca938: Retrying in X seconds (repeats countdown a few times with increasing wait times )
0ae30075c5da: Retrying in X seconds
9da81141e74e: Retrying in X seconds
b2e41dd2ded0: Waiting
7f40e809fb2d: Waiting
758848c48411: Waiting
23ded5c3e3fe: Waiting
38a847d4d941: Waiting
docker: received unexpected HTTP status: 503 Service Unavailable. (finally)
See ‘docker run --help’.
Thanks.
JS

Answer 1

Answering my own question: I ran docker through fiddler, by setting the docker proxy to localhost:8888. Inspecting the body of the 503 response in fiddler, I was able to see the cause: The corporate firewall rules were blocking executable file transfers.