As mentioned here:
Email and Logger are included as part of base NZBGet, but they are not included in the latest nzbget Docker image (as of 2019-11-30).
I “docker exec bash-ed” into the filesystem and didn’t see them, nor do they show up in the NZBGet Settings when browsing Extensions within Settings -> Extension Scripts -> Choose.
I have NZBGet installed on a non-Docker Linux system elsewhere and they are included within the installation. They appear within NZBGet’s GitHub as well, when browsing in the source tree.
This Docker install was on a Synology DS1512+, is running several other Docker images from LinuxServer.io without issues.