Also need this. For now, I'm using with-ssh-docker-socket (disclaimer: i built that). It lets you specify the SSH command using go-templates, e.g.:

$ with-ssh-docker-socket \
    -ssh-app='ssh -nNT -L "{{.LocalPort}}:{{.RemoteSocketAddr}}" "{{.RemoteHost}}"' \
    -a user@remote-host \
    docker ps # (or any other command, e.g. docker-compose)

Also there's an -ssh-app-extra-args flag that maps to the .ExtraArgs template variable, which is used in the preset templates (-ssh-app-openssh, -ssh-app-putty).

I would prefer to use docker's built-in SSH support, but having to re-write ~/.ssh/config is a deal-breaker for me, hence the wrapper approach until this lands.

Just adding another use case here.

I've been looking at trying to split up my terraform modules into two main parts: instance and container. This allows me to separate concerns and only update/rebuild my container when absolutely need-be.
Anyways, this feature (mainly passing a file for ssh keys and furthermore, allowing connection through a Bastion box) would allow me to SSH into the boxes that I'm building with terraform.

Just a small additional use case :)

We have no plans at the moment to extend the scope of this feature. So, I'm closing this issue for now but we can possibly reconsider in the future.