To trust a self-signed certificate from a Docker Trusted Registry (DTR), you need to place the certificate in the appropriate location on all cluster nodes and restart the Docker daemon. There are two possible locations for the certificate, depending on your OS and Docker version1:

•/etc/docker/certs.d/dtr.example.com/ca.crt: This is the preferred location for Linux systems and Docker versions 1.13 and higher. This directory is scanned by Docker for certificates and keys for each registry domain2.

•Your OS certificate path: This is the fallback location for other OSes and Docker versions. You need to find the certificate store for your OS and copy the certificate there. You also need to trust the certificate system-wide, which may require additional steps depending on your OS3.

The other options are not correct because:

•Passing '-trust-certificate ca.crt to the Docker client is not a valid option. There is no such flag for the Docker client4.

•Placing the certificate in ‘/etc/docker/dtr/dtr.example.com.crt’ is not a valid location. The certificate should be in the /etc/docker/certs.d directory, not the /etc/docker/dtr directory1.

•Passing – insecure-registry to the Docker client is not a recommended option. This flag disables the TLS verification for the registry, which makes the communication insecure and vulnerable to attacks.