Add rootless container builds via Docker/Podman

[atc0005]

• update builder image Dockerfile
  • copy project/repo content into container at build time - explicitly changing owner:group to builduser
  • explicitly create new builduser user and group
    • this is explicitly used for Docker-based builds
  • set /builds as the working directory
  • add new .dockerignore file to exclude the same items as the .gitignore file
  • set Git safe.directory logic at system level
• update Makefile recipes
  • add separate docker/podman variants of container-based project build recipes - each uses slightly different logic to achieve rootless container execution
  • explicitly emit the tool used to perform specific tasks
    • this can be useful to help explain why a generated builder image does not appear in the docker image ls output as a sysadmin might expect (if it was instead built with the docker command)
  • rename/remove the helper build recipe from the help recipe output (not useful to call directly)
  • to explicitly run the build container as the builduser user that is created during build image generation when using Docker to build/run containers (Podman uses different settings)
  • to send xz compressed output to stdout, then redirect to a target file
    • this works around failures to chmod and chgrp the compressed copy of input files when run within a non-root container
  • to explicitly bind mount the release_assets path into /builds/release_assets (using the same Makefile variable) read/write (instead of relying on implied read/write access)
  • to explicitly use /builds as the working directory

This collection of changes allows reliably building this project using either Docker or Podman via a "rootless" container.