Add Dockerfiles for Debian Trixie image that still uses the Bookworm deb version

[omnivelociraptor]

This change...

• copies the existing files Dockerfile, Dockerfile.buildpack and Dockerfile.fat from bookworm into a new directory trixie for Debian
• sets the first RESTY_IMAGE_TAG arg value to "trixie" in Dockerfile and Dockerfile.tag, or "trixie-slim" in Dockerfile.buildpack
• updates the echo commands to use the new deb822 format when adding the openresty apt repo to sources.list.d/debian.sources
• adds Trixie 'flavor' to .travis.yml, BUILDING.md, README.md and Taskfile.yml files

Caveats

• it still uses the previous image Bookworm deb package version, 1.27.1.2-1~bookworm1, until next release