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

imagefiles/dockcross

@@ -193,6 +193,16 @@ else
     HOST_PWD=$PWD
 fi
 
+# Mount Additional Volumes
+if [ -z "$SSH_DIR" ]; then
+    SSH_DIR="$HOME/.ssh"
+fi
+
+HOST_VOLUMES=
+if [ -e "$SSH_DIR" ]; then
+    HOST_VOLUMES+="-v $SSH_DIR:/home/$(id -un)/.ssh"
+fi
+
 #------------------------------------------------------------------------------
 # Now, finally, run the command in a container
 #
@@ -200,6 +210,7 @@ tty -s && TTY_ARGS=-ti || TTY_ARGS=
 CONTAINER_NAME=dockcross_$RANDOM
 docker run $TTY_ARGS --name $CONTAINER_NAME \
     -v "$HOST_PWD":/work \
+    $HOST_VOLUMES \
     $USER_IDS \
     $FINAL_ARGS \
     $FINAL_IMAGE "$@"