Add nightly deployment config

[laymonage]

• Add a NIGHTLY build arg to the Dockerfile that would replace the wagtail dependency in requirements/base.txt with a link to the nightly wheel.
• Add a GitHub Actions workflow configuration that:
  • checks out the main branch,
  • sets the NIGHTLY build arg in heroku.yml to 1
  • commits and pushes to the nightly branch (with --force to ensure it's up to date with latest main)

Heroku will pick up the push to the nightly branch and deploy it to our nightly demo app. We don't set NIGHTLY: 1 directly in heroku.yml in main because we also deploy the demo with the latest stable version of Wagtail as defined in requirements/base.txt.

[laymonage]

I've already tried the Dockerfile in a manually created nightly branch and confirmed that it works: 6096ae0

I'm going to merge this and hope the CI works 🙏

[laymonage]

Aand it works! 🥳 🎉

https://github.com/wagtail/bakerydemo/commits/nightly/