Deal with more `docker-compose.yml` corner cases

[emk]

There are some corner cases in docker-compose.yml that we should handle better.

• nil instead of structs (especially under volumes)
• integers and booleans instead of string values in environment and driver_opts
• variable interpolation in various Map structures

It's easier to write the ugly serde deserializers for all of these than it is to explain the restrictions to people.

[emk]

Moved to https://github.com/emk/compose_yml/issues/3, https://github.com/emk/compose_yml/issues/4, https://github.com/emk/compose_yml/issues/5.