Like custom build functionality (custom Dockerfile), adds support for
custom docker-compose.yml functionality:
* Adds default custom compose file
* Adds to run-time environment
* Adds to `gen` and `edit`
- Since these files are only ever sourced, deduction must be relative to
`BASH_SOURCE` (or else realpath will break)
- Fixes client-side `plugins` completion
* realpath breakage was introduced in #301
