Enable cloning via ssh

[slinstaedt]

The current plugin has some issues, when it comes to clone a repository via ssh instead of http(s):

1. As the ssh key is passed as environment variable and line breaks might need to be escaped accordingly passing the key into id_rsa with printf instead of echo is appropriate.
2. Not all git servers use the default ssh port (22) for accessing the git repos (looking at you, stash). Afaik the port is only encoded into DRONE_GIT_SSH_URL, so some parsing magic is necessary to extract the information and trigger ssh-keyscan with the correct parameter. Also DRONE_NETRC_MACHINE is not always matching the ssh host.
3. Replace DRONE_REMOTE_URL with DRONE_GIT_SSH_URL, so follow up scripts does not need to be touched.

[slinstaedt]

PS: Please squash, if PR is accepted. I linked the forked repo with docker hub to get automated images while testing out various problems using good old console logging ;)