EDIT:
I missread your post an thought you weren't able to set the new variable but that is not the case. I will leave my response here for others who might need it.
I am using InfluxDB as Database, because I had it already installed.
Hey,
just follow the directions for Docker Containers on the linked page:
Open the CLI of the Speedtest Docker container
Run
php artisan key:generate --show
This will create a key starting with base64:
Add the generated string to your environment variables attached to APP_KEY=generatedstringgoeshere (paste the key including the 'base64:' in the beginning)
Restart the container
For Step 3 I added an additional variable in the speedtesttracker config:
Add another Path, Port, Variable, Label or Device -> config type: Variable
Or you reinstall the Container because @ZappyZap added the variable already.
Cheers