Update Docker tests to use --headless

Author: mdmintz

docker/run_docker_test_in_chrome.sh

@@ -2,5 +2,5 @@
 set -e
 # Run example test from inside Docker image
 echo "Running example SeleniumBase test from Docker with headless Chrome..."
-cd /SeleniumBase/examples/ && nosetests my_first_test.py --config=docker_config.cfg --browser=chrome
+cd /SeleniumBase/examples/ && nosetests my_first_test.py --config=docker_config.cfg --browser=chrome --headless
 exec "$@"

docker/run_docker_test_in_firefox.sh

@@ -2,5 +2,5 @@
 set -e
 # Run example test from inside Docker image
 echo "Running example SeleniumBase test from Docker with headless Firefox..."
-cd /SeleniumBase/examples/ && nosetests my_first_test.py --config=docker_config.cfg --browser=firefox
+cd /SeleniumBase/examples/ && nosetests my_first_test.py --config=docker_config.cfg --browser=firefox --headless
 exec "$@"