LWT_TOPIC missing

[Bolukan]

Describe the bug
Setting the environmental variable LWT_TOPIC does not change the used LWT topic.

Could you add the LWT_TOPIC which default is home/TheengsGateway/LWT to the supported environment variables as I prefer a different topic.

Thx

To Reproduce
Add LWT_TOPIC as variable to the composer file:

Only relevent line added:
environment:
LWT_TOPIC: ble/theengs/LWT

Restart container and check the MQTT output: nothing changes.

Expected behavior
The LWT_TOPIC can be set.

Additional context
The docker file is not processing the variable and needs an update to include LWT_TOPIC as possible parameter.

[Bolukan]

I wrote the needed change: theengs/gateway-docker#6

I did some checks to comply to the quality of the current code:
Like other mqtt topics it is a free text fields and no checks are to be performed, only add it to the config file.
Second, I looked into the config file of a running instance to add the line to the same place.