Possible extension for e-mail/mta/mail transport agent docker setup instructions

[cfi-gb]

This is a possible extension for #349:

In https://stackoverflow.com/questions/78651310/maintainable-openvas-installation-with-email-support-in-aws#comment138693636_78651310 it was mentioned the following:

Albeit i have to apt install ca-certificates, otherwise i get TLS cert check errors (and i don't want to use the disable TLS check env var).

which might need an extension of the instructions here:

• https://greenbone.github.io/docs/latest/22.4/container/workflows.html#setting-up-a-mail-transport-agent-inside-docker-container
• https://github.com/greenbone/docs/blob/main/src/22.4/container/workflows.md#setting-up-a-mail-transport-agent-inside-docker-container