The images at https://images.linuxcontainers.org are built by our Jenkins server at https://jenkins.linuxcontainers.org using the scripts found at https://github.com/lxc/lxc-ci and the LXC template scripts from https://github.com/lxc/lxc-templates. That latter part is changing as we’re moving those images to using distrobuilder too, right now Debian and Ubuntu have been switched over with the distrobuilder yaml files available in the lxc-ci repository.
build-lxc is indeed equivalent of build-dir + pack-lxc, most users won’t need to use the build-dir action. That action was added mostly for our own needs where we build and publish images for both LXC and LXD.
Config customization for LXC should be done in /var/lib/lxc/CONTAINER-NAME/config as was the case before.
If you want a change to affect all newly created containers you may put it in /etc/lxc/default.conf.
If there’s something you wish all your containers to have (existing and future), you can put it in a file in /usr/share/lxc/config/common.conf.d/