docker: ensure socat is installed

install_docker.sh failed for me because socat wasn't installed.  Add it
to this script since it expects it to be there.

Change-Id: Ic55f5e38de1b38bdd37407b7bec533d4c3eff2a9
diff --git a/tools/docker/install_docker.sh b/tools/docker/install_docker.sh
index d659ad1..289002e 100755
--- a/tools/docker/install_docker.sh
+++ b/tools/docker/install_docker.sh
@@ -38,7 +38,7 @@
 install_package python-software-properties && \
     sudo sh -c "echo deb $DOCKER_APT_REPO docker main > /etc/apt/sources.list.d/docker.list"
 apt_get update
-install_package --force-yes lxc-docker=${DOCKER_PACKAGE_VERSION}
+install_package --force-yes lxc-docker=${DOCKER_PACKAGE_VERSION} socat
 
 # Start the daemon - restart just in case the package ever auto-starts...
 restart_service docker