HTTPS integration with self-signed server certificate

I got the following error message:

chirpstack-application-server ... error="http request error: Post \"https://...?event=up\": x509: **certificate signed by unknown authority**" ...

Question is - where to add a CA certificate?
I tried /usr/local/share/ca-certificates/… - not working.

Is it on system level or web client of Application Server used something different?

BTW. I run Docker composition of everything.