Requires linuxserver/docker-documentation#32 to be merged

Ref: 92c6e348cffd05ff4fd78c038f43877e7886ff85

Use expansion instead of subshells
Use latest tag if tag is not set

custom services are now copied into easily identifiable directory so we can remove and 'start fresh' every container run. This allows users to remove services and the changes to take effect on a restart rather than having to rebuild the container

adding pre-init modification logic to pull a tarball of files from dockerhub and apply it to the image if the user passes DOCKER_MODS env variable