Add build argument to Dockerfiles for release version

[dolsysmith]

Modify the Dockerfiles to use an optional build argument, which can be passed in via docker-compose. The build argument would have a default value of release, but it could be set to master in cases where we want the images to be the latest in the repository (and not those from the latest release).

[dolsysmith]

Assuming the goal is for someone to be able to run the latest version on master without building the images from the various repos, the solution is to pass the arguments to the builds in Dockerhub. This can be done, apparently, with a build hook script, which would pass the correct build argument based on the Docker tag (i.e., master for the latest code in master, and the appropriate release tag otherwise).

[dolsysmith]

This is working in sfm-twitter-harvester. Before implementing the hook in the other repos (including sfm-ui), need to confirm that the updated version of pytz in sfm-utils is working with the code in those repos.