Updates docker-compose for local development

[dleehr]

Updates docker-compose to simplify local development

• Uses compose file version 3
• Mounts source tree into /usr/src/app, preserving settings.docker
• Configures web container to build locally instead of pulling from docker hub (as docker hub is being phased out of this deployment)
• Adds docker-compose service to run background task processing
• sets postgres data volume to be a docker named volume