Pin base python package requirements

- Require invoke to be installed before we can run "invoke update"

docker/requirements.txt

@@ -0,0 +1,13 @@
+# Base python requirements for docker containers
+
+# Basic package requirements
+setuptools>=57.4.0
+wheel>=0.37.0
+invoke>=1.4.0                   # Invoke build tool
+gunicorn>=20.1.0                # Gunicorn web server
+
+# Database links
+psycopg2>=2.9.1
+mysqlclient>=2.0.3
+pgcli>=3.1.0
+mariadb>=1.0.7