There are many different resources:

Basics:

• Archwiki Basics: Creating Packages
• Archwiki Developerwiki: Building in a clean chroot -- useful for building on normal machine, not as useful for cicd/docker development
• Archlinux base docker image official (160mb)
• Pacman basics: Using a custom (local) repository

PKGBUILD and Archrepo explanation:

• 3 parter: Hosting arch repos on s3, managing archlinux with metapackages, creating a custom installer
  • example repo of archpkgs
  • example archlinux install script
  • very basic example pkgbuild
• additional advanced ways of building replicating metapkgs
• older article: building packages on linux
• older article: creating an archlinux repository

PKGBUILD examples:

• extensive example archrepo on github
• another example archrepo

CI/CD:

• extensive automated builds -- fetching from different repositories and automatically hosting on gitlab pages (very useful resource); awesome documentation
• more minimal setup -- using 1 repository for packages; mainly bash building, does not deploy a gitlab pages repo (can be adapted from above link); nicely documented
• in-between -- look at ci-library.sh/ci-build.sh; more well-established practices than repo above; but almost-nonexistent documentation