The Problem
Sometimes, and in the commercial WP plugin world pretty often, a plugin may depend on code that comes from a private repository. Or maybe you have 2FA and SSH set up in your Github account, which now tracks usage quotas. In these cases, SSH will need to use your key in order to authenticate with the repo server. These are not available in the Docker image, and thus no installation of said dependencies is possible.
Solution
Use runtime SSH agent forwarding. It works very well and keeps the container clean of keys. It works by mounting the host's SSH agent socket into the container, and forcing the container to use that socket. Note that the most compatible path to the socket, and thus a good default value for both source and destination, on both Linux and Mac, is /run/host-services/ssh-auth.sock.
The Problem
Sometimes, and in the commercial WP plugin world pretty often, a plugin may depend on code that comes from a private repository. Or maybe you have 2FA and SSH set up in your Github account, which now tracks usage quotas. In these cases, SSH will need to use your key in order to authenticate with the repo server. These are not available in the Docker image, and thus no installation of said dependencies is possible.
Solution
Use runtime SSH agent forwarding. It works very well and keeps the container clean of keys. It works by mounting the host's SSH agent socket into the container, and forcing the container to use that socket. Note that the most compatible path to the socket, and thus a good default value for both source and destination, on both Linux and Mac, is
/run/host-services/ssh-auth.sock.