build-unit-test-docker: Add socat

Adding possibility to use socat in the SSH configuration.

Resolves openbmc/openbmc-build-scripts#36

Tested:
run-unit-test-docker.sh runs with EXTRA_DOCKER_RUN_ARGS=
"-v /home/user/.gitconfig:/home/user/.gitconfig
-v /home/user/.ssh:/home/user/.ssh"

Change-Id: I45e9279c5e3bd3bfc59113bc41ba2abfd7eb1bfa
Signed-off-by: Ewelina Walkusz <ewelinax.walkusz@intel.com>
diff --git a/scripts/build-unit-test-docker b/scripts/build-unit-test-docker
index 3011c2f..7bc1157 100755
--- a/scripts/build-unit-test-docker
+++ b/scripts/build-unit-test-docker
@@ -823,6 +823,7 @@
     python3-yaml \
     rsync \
     shellcheck \
+    socat \
     sudo \
     systemd \
     valgrind \