The git clone plugin cannot be used in a generic way to clone other repositories. If you want to clone additional repositories, you can add a clone step to your pipeline commands:
Note that you do not need to use secrets here. If your main repository is private, drone will automatically create a netrc file in your containers which git will use by default to automatically authenticate when cloning. Note that you must use the git+http clone urls for this to work.
+ git clone git@bitbucket.org:someteam/some-repo.git
Cloning into 'some-repo'...
Host key verification failed.
fatal: Could not read from remote repository.
Please make sure you have the correct access rights
and the repository exists.
https links are not ok in bitbucket, they contain username in them, and they failed also anyway.
+ git clone https://myname@bitbucket.org/someteam/some-repo.git
Cloning into 'some-repo'...
fatal: could not read Password for 'https://myname@bitbucket.org': No such device or address
/drone/src/bitbucket.org/someteam/somerepo # git clone git@bitbucket.org:someteam/some-repo.git
Cloning into 'some-repo'...
The authenticity of host 'bitbucket.org (104.192.143.2)' can't be established.
RSA key fingerprint is SHA256:zzXQOXSRBEiUtuE8AikJYKwb-bla-bla.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added 'bitbucket.org,104.192.143.2' (RSA) to the list of known hosts.
Permission denied (publickey).
fatal: Could not read from remote repository.
Please make sure you have the correct access rights
and the repository exists.
I do something like this in one of my private repositories and can confirm it works without issue. However, if for some reason you cannot get it working, you can always fallback to an ssh key stored in a secret. I do this too in one of my projects, see this example.