Add Docker Mount of SSH hidden directory to docker installation for M…

[savtrip]

As per this issue here: basecamp/kamal#218

This exposes the hidden .ssh directory to the docker container. Solved by @gioggi and @stewones. I spent at least 1 hour banging my head wondering why my ssh auth wasn't working. I forgot I was using a Docker container and it wasn't exposing my Mac's SSH keys, I found the linked GitHub issue that made me aware of the issue.

I am happy to discuss any changes.

[strzibny]

Isn't the right thing to add the key with ssh-add to the agent?

[savtrip]

Hey @strzibny thanks for the comment, after further reading I do agree with you, that is the recommend way in the Docker docs (as kamal is already doing in its command in the docs): https://docs.docker.com/desktop/networking/#ssh-agent-forwarding

Instead of closing my pull request I am happy to revise it to leave a note to other people to ensure they load their generated SSH keys to the SSH agent. Perhaps a code example in a note adjacent to the section where it shows the aliases.

Also @strzibny thank you for your book on Kamal, I bought and read it. It had excellent practical examples and flowed well, thanks for putting it together.

[strzibny]

Yes, I would add a note about loading the keys, perhaps with a link to Docker docs :) and thanks! :)