Link to alternative build of Central

[matthew-white]

In getodk/central#22, there was discussion of alternative deployments/builds of Central. One nice result from that conversation was https://github.com/bastiondev/odkcentralcontainer. As described in the readme:

This repository builds a single Docker image for ODK Central with its configuration driven from environment variables, appropriate for serverless environments. It is also configured without SSL, to allow using an external load balancer that handles HTTPS termination and certificate management.

It does not include a Postgres Instance, and assumes an external database will be used to allow ODK Central to operate in a multi-node configuration.

The target environment is Amazon Web Services' Elastic Container Service and Heroku.

This issue is to link to this alternative build from the user docs.