Use lighter postgres image for tests

Author: WhyNotHugo

tox.ini

@@ -21,7 +21,7 @@ passenv =
   GENTESTCSR
 
 # Hint: quickly run a one-shot container with:
-#   docker run --rm -e POSTGRES_PASSWORD=postgres -p 5432:5432 -it postgres
+#   docker run --rm -e POSTGRES_PASSWORD=postgres -p 5432:5432 -it postgres:alpine
 [testenv:live]
 extras = dev, postgres
 commands = pytest -vv -m "live" {posargs}