Using custom TextFSM templates

First, place your TextFSM index and templates in a directory on the host machine.

Then, edit the x-api section in docker-compose.override.yml by 

• Adding a new environment variable NET_TEXTFSM
• Adding a new volume mapping

example:x-api: &api_common
  environment:
    NET_TEXTFSM: /textfsm
  volumes:
    - <your-template-dir>:/textfsm  

This is how Netmiko will find the templates. Of course – the template directory has to contain all TextFSM templates, not only the customized / your own ones.