By managing volumes in a better fashion and using code that is closer
to being idempotent, while being declarative, we achieve an image that
is closer to the original one, but gets the plugins that we want and
the configuration that we want for integration with DD.
Closes#9. This image now allows for BBB_HOST and BBB_API_SECRET as
variables in dd.conf, which also configure the corresponding plugin on
Nextcloud.
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