Reduce docker image building frequency and add default user/shell into docker image

Makes the docker containers more useful by providing a default user and shell. This could conceivably be problematic for downstream containers (the Dockerfile commands will run as this user). I'd be happy to consider removing this but it's hard to imagine users using these containers if they get sh as root by default.

Also reduces build frequency. It's hard to imagine builds on every push being a useful use of space.