Certificate verification error when adding a repository

[Romain DELAMARE <romain.delamare@legrand.fr>]

Hello,

I am trying to build an custom ISO of Debian buster that includes the official Docker repository. I have followed the instructions of the Debian live manual (in particular section 8.1.5) and of the Docker documentation (https://docs.docker.com/engine/install/debian/):
    - I have added a "docker.list.chroot" file containing the repository ("deb [arch=amd64] https://download.docker.com/linux/debian buster stable")
    - I have added a "docker.key.chroot" file containing the PGP public key of the Docker repository

When I try to build the ISO, I get an error:
"Err:6 https://download.docker.com/linux/debian buster Release
  Certificate verification failed: The certificate is NOT trusted. The certificate issuer is unknown.  Could not handshake: Error in the certificate verification. [IP: 2600:9000:215b:a800:3:db06:4200:93a1 443]"

Am I doing something wrong here ? Or should I do more ? I could not find more information about this issue.

Thanks,
Romain Delamare

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