Support and document the use of env-vars for options

```

Environment Variables

---------------------

Any of the configuration values may also be passed as environment variables 

to the container, however:

- Unlike the way most configuration systems treat environment variables, 

  they do not overwrite options provided in the configuration files unless 

  the files are specifically written to honour the environment variables.

- For array options the value provided in the environment variable will 

  become the first item (index 0).

Convenience Files

-----------------

declare -r DEFAULT_THEME=twentynineteen

declare -a THEMES=() PLUGINS=() LANGUAGES=()

declare DB_HOST DB_NAME DB_USER DB_PASS

declare -a THEMES=( ${THEMES-} )

declare -a PLUGINS=( ${PLUGINS-} )

declare -a LANGUAGES=( ${LANGUAGES-} )

declare -a STATIC_PATTERNS=(

	"*.crt"

	"*.md"