docker: add required env var

newer versions of the postgres images require the default password to be
explicitly set via an env var

Signed-off-by: Petros Angelatos <petrosagg@gmail.com>

Author: petrosagg

docker-compose.yml

@@ -1,6 +1,9 @@
 version: '2'
 services:
   postgres:
-    image: "sfackler/rust-postgres-test:6"
+    image: "sfackler/rust-postgres-test:7"
+    build: ./docker
+    environment:
+      - POSTGRES_PASSWORD=password
     ports:
     - 5433:5433