Suggestion: Add pyln-client python module to c-lightning docker image

[ksdhans]

BTCPay sets up c-lightning to have a plugins directory in a docker volume. This is useful. However, many of c-lightning's plugins need pyln-client which, AFAICT, needs to be installed in the container itself. Pyln-client has other dependencies, some of which must be built using GCC (with some *-dev dependencies also installed).

It might be a good idea to have pyln-client installed by default. That way it's immediately available, and won't disappear if the container is refreshed.

[ksdhans]

Looking at this again, I think it's almost a necessity. Almost all c-lightning plugins are Python based, and depend on pyln-client. So, copying the plugin to the plugins directory in the docker volume, won't work.

Added to that, the c-lightning docker entrypoint.sh overwrites the config file, which means you can't customize the settings to enable sqlite database backup, for example.