Lock docker image versions

[wwsean08]

To prevent issues and differences between deployments, lock the version of postgres/redis to specific versions. Currently postgres is using latest, and redis is using the latest alpine image. This would help with difficult to reproduce issues down the line potentially.

[FiniteSingularity]

Good thought. We're right in the middle of overhauling the docker container setup, see issue #35 . I've added this to the list in that issue.