gin-contrib session testing randomly fails on gitlab ci

Question

I am using gin-contrib sessions and reproduced the tests of this example test file: https://github.com/gin-contrib/sessions/blob/98372e3e8562a0d4b3d319db0d79ffbf5898e360/tester/tester.go

In contrast to the example, I set some session options in my login handler:

func (h *handler) login(c *gin.Context, u model.Participant) {
    // [sign in code reduced]
    dbuser := u.Login(h.db, c)

    // start session
    session := sessions.Default(c)
    session.Options(sessions.Options{
        MaxAge:   3600,
        Path:     "/",
        HttpOnly: true,
        Secure:   true,
    })
    session.Set("id", dbuser.ID)
    session.Set("name", dbuser.Name)
    if err := session.Save(); err != nil {
        c.HTML(http.StatusBadRequest, "index.html", gin.H{"error": err.Error()})
        return
    }

    c.Redirect(http.StatusFound, "/dashboard")
}

Now I wrote a test for this handler:

// LoginCookie
// This function is only needed for test purposes
func LoginCookie(router *gin.Engine, t *testing.T) []string {
    t.Log("performing login...")
    w := httptest.NewRecorder()
    req, _ := http.NewRequest("POST", "/login", strings.NewReader("name=bob&password=j"))
    req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
    router.ServeHTTP(w, req)

    assert.Equal(t, 302, w.Code)
    assert.Equal(t, "/dashboard", w.Header().Get("location"))
    assert.Equal(t, true, strings.Contains(w.Header().Get("Set-Cookie"), "mysession"))
    t.Log("login successful")
    return w.Header().Values("Set-Cookie")
}

This function should sign in a user defined in the fixtures. It works locally, but randomly fails as soon as I try to run it in a gitlab ci.

Here is my gitlab ci script:

format:
  stage: test
  services:
    - name: harbor.uni-muenster.de/proxy-docker/library/postgres:16
      alias: postgres
  variables:
    POSTGRES_DB: cvmlsannotationserver_test
    POSTGRES_USER: cvmlsannotationserver_test
    POSTGRES_PASSWORD: cvmlsannotationserver_test
  image: harbor.uni-muenster.de/proxy-docker/library/golang:latest
  script:
  - go fmt $(go list ./... | grep -v /vendor/)
  - go vet $(go list ./... | grep -v /vendor/)
  - ANNOTATIONSERVER_DSN=postgres://cvmlsannotationserver_test:cvmlsannotationserver_test@postgres:5432/cvmlsannotationserver_test?sslmode=disable go test -race $(go list ./... | grep -v /vendor/)

The fixtures are being loaded, the user should be able to log in, but the test randomly sometimes passes and sometimes fails.