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.manylinux

@@ -23,5 +23,5 @@ RUN /usr/local/bin/install-python-packages.sh
 
 COPY manylinux-common/pre_exec.sh /dockcross/pre_exec.sh
 
-RUN yum -y install pax zip \
+RUN yum -y install pax zip openssh-clients \
   && yum clean all