docker-compose.yml: Use postgresql:alpine image

This (the Alpine Linux-based postgresql image) is a smaller image
than the debian-based postgresql image,
so it should be marginally faster to download.

Seems like a good idea in general,
to speed up build times (even outside of Travis CI).

Also should save some disk space for developers.

--

Inspired by the general concept of @btyy77c's dockerAlpine branch:
  - https://github.com/btyy77c/refugerestrooms/commits/dockerAlpine

Docker Hub documentation on the alpine vs debian postgres images:
  - https://hub.docker.com/_/postgres#image-variants

Author: DeeDeeG

docker-compose.yml

@@ -1,7 +1,7 @@
 version: '3'
 services:
   db:
-    image: postgres
+    image: postgres:alpine
   web:
     build: .
     entrypoint: [setup/entry]