I am running ignition gateway as docker container and it is acting as client to communicate with external services.
When i run the gateway locally, I was able to avoid the error, "unable to find valid certification path to requested target" by placing a certificate to /ignition/data/certificates/supplemental.
Same for the docker container, I placed the certificate inside of the docker container /usr/local/bin/ignition/data/certificates/supplemental.
Upon starting the gateway, you should see feedback in the logs for the loading of your supplemental certificates into the trust store.
If successful, you should see something like:
Certificate Added to trust store: ...
Otherwise you should get a warning with more details:
Unable to add certificate xxxxx to trust store, ...
If you see neither of these related to your certificate, double check your placement--you've got the right folder though, /usr/local/bin/ignition/data/certificates/supplemental.