Allows SSH Agent forwarding into docker container

Docker-forward-ssh-agent.sh

# NB: the line with SSH_AUT_SOCK allows SSH Agent forwarding into docker container
# NB: umask 002 makes all newly created files group-writable
docker-run-with-agent: 
  docker run -t -i \
  -v `pwd`/source:/srv/docker-jekyll/source/ \
  -v `pwd`/deploy:/srv/docker-jekyll/deploy \
  -v `dirname $(SSH_AUTH_SOCK)`:`dirname $(SSH_AUTH_SOCK)` -e SSH_AUTH_SOCK=$(SSH_AUTH_SOCK) \
  -p 4000:4000 \
  dergachev/docker-jekyll \
  /bin/bash -c "umask 002; $(cmd) $(args)"

@vasi says this is terrifying