GPG error when trying to sign the docker repo

[pdemarti]

Describe the bug

In the task: "add docker repository to apt", there is a failure:

Failed to update apt cache: W:GPG error:
https://download.docker.com/linux/debian buster InRelease: The following signatures couldn't
be verified because the public key is not available: NO_PUBKEY 7EA0A9C3F273FCD8,
E:The repository 'https://download.docker.com/linux/debian buster InRelease' is not signed.

Possibly related to: this SO issue.

As a workaround, I changed the task to not check signature (potential vulnerability):

    - name: add docker repository to apt
      become: true
      apt_repository:
        # we won't try to sign the repo. If we do, we get: "Failed to update apt cache: W:GPG error:
        # https://download.docker.com/linux/debian buster InRelease: The following signatures couldn't
        # be verified because the public key is not available: NO_PUBKEY 7EA0A9C3F273FCD8,
        # E:The repository 'https://download.docker.com/linux/debian buster InRelease' is not signed."
        # repo: deb [arch=armhf signed-by=/usr/share/keyrings/docker-archive-keyring.gpg] https://download.docker.com/linux/debian buster stable
        repo: deb [arch=armhf] https://download.docker.com/linux/debian buster stable
        state: present