The helixbot user does not need to be set as the default user

[mthalman]

The Helix Dockerfiles should be updated to not set helixbot as the default user. They should be left with root as the default. This works because the Helix infrastructure will explicitly set helixbot as the user when running the container. And having it set to root by default will enable the container image to be used as a container job (see #1276 (comment)).