Cannot load Cypress test runner in 3.3.0 behind proxy

Application running in Docker, exposing http://localhost:3000.

Corporate proxy with exceptions given for localhost.

Error received:

Relevant debug log:

cypress:server:server Got CONNECT request from localhost:3000 +320ms
  cypress:https-proxy Writing browserSocket connection headers { url: 'localhost:3000' } +303ms
  cypress:server:cors Parsed URL { port: '3000', tld: 'localhost', domain: '' } +322ms
  cypress:server:server HTTPS request does match URL: https://localhost:3000 with props: { port: '3000', tld: 'localhost', domain: '' } +1ms
  cypress:https-proxy Not making direct connection { url: 'localhost:3000' } +1ms
  cypress:https-proxy Making intercepted connection to 63361 +0ms
  cypress:https-proxy making proxied connection { host: 'undefined:63361', proxy: 'http://proxyhostRedacted' } +0ms
  cypress:network:agent Creating proxied socket for https://undefined:63361 through http://proxyhostRedacted +307ms
  cypress:network:connect successfully connected { opts: { port: 8080, host: 'proxyhostRedacted', useTls: false, getDelayMsForRetry: [Function: getDelayForRetry] }, iteration: 0 } +318ms
  cypress:network:agent Proxy socket for https://undefined:63361 established +11ms
  cypress:https-proxy received upstreamSocket callback for request { port: 63361, hostname: undefined, err: Error: Error establishing proxy connection. Response from server was: HTTP/1.1 403 Forbidden

cypress.json:

So I noticed that cypress is trying to use a Proxy socket towards https://undefined, so I added undefined as an exception to the NO_PROXY environment variable in the shell that runs cypress, and everything started working again.

_Originally posted by @MichaelHindley in https://github.com/cypress-io/cypress/issues/4235#issuecomment-493857240_

[cypress-bot[bot]]

Released in 3.3.1.

[MichaelHindley]

@flotwig pastebin with debug logs without the undefined fix:

https://pastebin.com/rpNR5QEG