Occasional 504 Gateway Time-out when using api.dev.ooni.io

Describe the bug

While contacting https://api.dev.ooni.io, the API will occasionally return a 504 after 60 seconds.

To Reproduce

❯ cat data.json
{"charging":true,"on_wifi":true,"platform":"linux","probe_asn":"AS0","probe_cc":"XX","run_type":"manual","software_name":"miniooni","software_version":"3.23.0-alpha","web_connectivity":{"category_codes":[]}}
❯ curl -X POST -H 'Content-type: application/json' --connect-timeout 5 -d @data.json https://api.dev.ooni.io/api/v1/check-in
<html>
<head><title>504 Gateway Time-out</title></head>
<body>
<center><h1>504 Gateway Time-out</h1></center>
</body>
</html>

You need to send the request several times, frequency of the error varies. Some times it fails on check-in, others in submission, etc.

Expected behavior

Not a 504.

ooni / probe

Occasional 504 Gateway Time-out when using api.dev.ooni.io #2741