Super Tutorial!

It may have been some while and things moving may have upset the code!

I ran your repo and some tests worked but some did not. I really liked the filters.spec.js

I put some effort on sign.in.spec
https://cjdocs.herokuapp.com/auth/signin is very slow
so I changed globals.js and added an extra 0! waitForConditionTimeout: 100000

This version of signinPage.js worked apart for a non-show-stopper error

const signinCommands = {  
  signin(email, password) {
    return this
      .waitForElementVisible('@signinTop')
      .click('@signinTop')
      .waitForElementVisible('@emailInput')
      .setValue('@emailInput', email)
      .setValue('@passwordInput', password)
      .waitForElementVisible('@signinButton')
      .click('@signinButton')
      .click('@signinButton')
  }
};

export default {  
  url: 'https://cjdocs.herokuapp.com/auth/signin',
  commands: [signinCommands],
  elements: {
    emailInput: {
      selector: 'input[type=email]'
    },
    passwordInput: {
      selector: 'input[name=password]'
    },
    signinButton: {
      selector: 'button[type="submit"]'
    },
    signinTop: {
      selector: 'a[href="/signin"]'
    }
  }
};

Changes

1. Had to insert a signinTop
2. You may have missed quotes around submit
3. But .click('@signinButton') with only 1 does not work;
   2 does but gives a error that does not stop instancesPage code execution

One change of text in signin.spec.js Welcome to the Docs Home! no CJ

module.exports = {
  'User can sign in'(client) {
    const signinPage = client.page.signinPage();
    const instancesPage = client.page.instancesPage();

    signinPage
      .navigate()
      .signin(process.env.EMAIL, process.env.PASSWORD);

    instancesPage.expect.element('@homepageWelcomeTitle').text.to.contain('Welcome to the Docs Home!');

    client.end();
  }
};

OUTPUT
C:\winreactjs\e2e-testing-nightwatch>nightwatch --env local

[Signin Spec] Test Suite

i Connected to 127.0.0.1 on port 4444 (5888ms).
Using: chrome (81.0.4044.138) on Windows NT platform.

Running: User can sign in

√ Element <a[href="/signin"]> was visible after 74 milliseconds.
√ Element <input[type=email]> was visible after 112 milliseconds.
√ Element <button[type="submit"]> was visible after 50 milliseconds.
Error while running .getElementText() protocol action: stale element reference: element is not attached to the page document

√ Expected element <//div/h2> text to contain: "Welcome to the Docs Home!" (2178ms)

OK. 4 assertions passed. (7.353s)

Regards & Thanks
Jayanta@Kolkata