Fix docker permission error when generating config with the current synapse:latest image

[xddq]

Fixes small config error.

Currently if we are creating a fresh configuration for matrix (based on the :latest docker image and the suggested commands) and then run the image with the suggested commands, we get a permission error.

PermissionError: [Errno 13] Permission denied: '/homeserver.log'.

issues:

10023
9970

What changed?

• added the LOG_FILE_PATH env to the readme.md for the docker installation (documentation).
• changed the default path from /homeserver.log to /data/homeserver.log (configuration)
• I added a folder for you to easier test this issue. Be aware that running the script will delete the default volume that is described in the docker readme ("synapse-data").. I will remove this folder afterwards, if you think the change is okay.

[richvdh]

nb this duplicates #10045.

[richvdh]

thank you for the contribution, but #10045 has got there first :)