SSH Support for Git Clone / Checkout

Added SSH support to enable Git checkouts that use SSH instead of
HTTP(S).

The SSH directory is assumed to be in $HOME/.ssh; however:

export SSH_DIR=/my/custom/dir

will override the setting and allow for configurable settings.

$SSH_DIR is then mounted as host-volume in the Docker container and
placed in /home/<user>/.ssh

common.debian

@@ -24,6 +24,7 @@ RUN apt-get update --yes && apt-get install --no-install-recommends --yes \
   python-pip \
   rsync \
   sed \
+  ssh \
   bison \
   flex \
   tar \