Configuring SSL - Unable to verify the first certificate

I get this error when using Postman or other command line tools, when verifying SSL they respond “Unable to verify the first certificate”. In the Browser it is fine as I understand browser downloads CAs from the internet.

I have loaded all my CAs into Kong Manager under CA Certificates, but this doesn’t seem to do anything. I have also read that I need to combine my 3 CA certs into a CA-Bundle and define this as part of my docker compose setup. I am not sure then what the CA Certificates section is for in the Kong Manager UI then.

How do I correctly load CAs to Kong to ensure they are sent from Kong itself so I don’t get the error above?

I have tried to add in my docker compose

KONG_SSL: "on"
KONG_SSL_CERT_CAFILE: /usr/local/kong/ca-bundle.crt

Where ca-bundle.crt is a bundle of all the CA certs my provider gave me, but I still get “Unable to verify the first certificate”. Kong seems to accept the above configuration without any errors in the logs.

Any ideas how I can troubleshoot why CAs aren’t being sent from Kong itself?