allow users to pass a private ssh key to be loaded into earthly

[alexcb]

Allow a user to pass in one or more private ssh keys, which would be loaded into a new ssh-agent instance, and be made available to earthly.
This would allow users to link multiple private repos.

behind the scenes, it could be implemented with

eval "$(ssh-agent)"
echo "$key" | ssh-add -

Until this is supported, users can use https://github.com/webfactory/ssh-agent in addition to the earthly action