When building a Docker container (e.g. with build_and_push_containers.cmd), the following error is displayed in the command window:

ERROR: failed to solve [...] tls: failed to verify certificate: x509: certificate signed by unknown authority

This issue has been observed with Zscaler, but may also happen with other firewall/proxy services.

This service is interfering with the internet communication, which causes the certificate to be signed by an authority (the firewall service) that is unknown to docker.io and therefore cannot be verified.

There are several solutions:

1. Contact your IT to allow this network traffic through Zscaler unedited.
2. Try the steps explained on the Docker forum here or here.
3. Connect to a network that does not have Zscaler active.
4. Temporarily disable Zscaler so long as this does not compromise the security of your system.