Research using `docker build` as an optional deploy method

[jonpugh]

I think it is inevitable that we automate building containers and not just mounting volumes and setting environment variables.

We are already calling "docker-compose" and "docker" directly for things, so maybe we need to start using docker build in our process.

The reasons are:

• Some things are hard to do just by passing environment variables, for example setting configuration for things like the NGINX docroot. See #60
• We can avoid using host-mounted volumes so we can spread across machines without hassle.

We would need to maintain the option of using mounted volumes for development purposes, so that edits to code are visible live in the container.