Ship a circleci system user

[FelicianoTech]

The Next-Gen images were suppose to ship with just a root user. That was the plan.

The security gains by using a regular user with sudo access are irrelevant in the world of Docker because:

1. With sudo they get root privileges anyway
2. With passwordless sudo, there's no additional security of a password.

Due to legacy image migration concerns, having sudo around would be useful. I'm not sure if useful enough on its own, but there are CircleCI platform features that expect to see a circleci home directory.

With little negative affect to our images and lots of positive affect for migration, we should reintroduce the circleci user to images.

[eddiewebb]

Other impact besides sudo is anyone caching directories in user's home are restored as an absolute path will all be misses when they move from /home/circleci to /root