Support for rsa-key-size variable

[DominikTrenz]

I would like to set the key-size in the config file. The corresponding certbot option:
--rsa-key-size 4096

[OdinVex]

This I'm definitely interested in too.

[OdinVex]

PR was closed without any reason posted but it worked (just had to start with a clean setup). You can see I left instructions on how to modify your docker compose to hijack the entrypoint and make it default to 4096 (again for new installations).