Pass arbitrary GPG key to sign a repository

[Totktonada]

I have a GPG key, which is stored in an environment variable. I want to instruct mkrepo to use it to sign a repository (as for APT as well as for YUM repository).

Now it is not quite obvious, how to do so: I should add the key manually using some gpg command and somehow ensure that it is a default key.

That would be nice to access a key from an environment variable.

This is NOT about signing a package.