Mount the entire user SSH directory into build container

This enables the user to use an key file format (RSA, ED25519, ...).
Additionally, it allows more complex workflows (such as different SSH keys for
specfic sites, such as Github or Bitbucket), since the .ssh/config
file is also mounted into the container.

Fixes serverless#488

Author: jacksgt

README.md

@@ -71,7 +71,7 @@ custom:
     dockerSsh: true
 ```
 
-The `dockerSsh` option will mount your `$HOME/.ssh/id_rsa` and `$HOME/.ssh/known_hosts` as a
+The `dockerSsh` option will mount your `$HOME/.ssh/` directory as a
 volume in the docker container. If your SSH key is password protected, you can use `ssh-agent`
 because `$SSH_AUTH_SOCK` is also mounted & the env var set.
 It is important that the host of your private repositories has already been added in your

lib/pip.js

@@ -204,9 +204,7 @@ function installRequirements(targetFolder, serverless, options) {
       // Mount necessary ssh files to work with private repos
       dockerCmd.push(
         '-v',
-        `${process.env.HOME}/.ssh/id_rsa:/root/.ssh/id_rsa:z`,
-        '-v',
-        `${process.env.HOME}/.ssh/known_hosts:/root/.ssh/known_hosts:z`,
+        `${process.env.HOME}/.ssh/:/root/.ssh/:z`,
         '-v',
         `${process.env.SSH_AUTH_SOCK}:/tmp/ssh_sock:z`,
         '-e',