Add abililty to use system python

Moving virtual environments between systems is problematic... and
unnecessary when creating container images. These modifications allow
the ability to install app dependencies into the system python, and use
that instead of a virtual environment.

script/server

@@ -14,8 +14,13 @@ set -e
 # Ensure we are in the app root directory (not the /script directory)
 cd "$(dirname "${0}")/.."
 
+if [ -z "${SKIP_PIPENV+xxxx}" ]; then
+  CMD_PREFIX='pipenv run '
+fi
+PYTHON_CMD="${CMD_PREFIX}python"
+
 # Launch the app
-pipenv run python app.py ${@} &
+${PYTHON_CMD} app.py ${@} &
 child=$!
 
 wait $child