fetch drops session data

[jdfadams-msp]

I have an integration test that logs in to my web app, visits a search page, and tries to search.

Test fragment:

cy.visit('/search.php')
cy.get('[name=search_term]')
  .type('Gauss')
cy.contains('Go')
  .click()  // triggers the javascript below
cy.contains('Carl Friedrich Gauss')
  .should('have.prop', 'href')

Javascript:

const form_data = new FormData(form);
fetch('ajax/search.php', {
    body: form_data,
    method: 'POST',
}).then(function(response) {
  // do stuff
});

ajax/search.php authenticates the session before searching.

When I run this test in the cypress desktop app, the request arrives at ajax/search.php with session data, and the test passes.

When I run this test in a cypress docker container, the request arrives at ajax/search.php without session data, and the test fails.

When I run this test in a cypress docker container but add credentials: 'include' to the fetch options, the request arrives at ajax/search.php with session data, and the test passes.

When I run this test in a cypress docker container but replace fetch with equivalent jQuery,

const data = $(form).serializeArray();
$.post('ajax/search.php', data).done(function(data) {
  // do stuff
});

the request arrives at ajax/search.php with session data, and the test passes.

Why does the POST via fetch drop session data when the test runs in a cypress docker container? Does cypress hook into fetch in such a way that the request appears to be to a different domain?