fix(ci): add default environment variables to docker-compose for CI

The docs validation was failing in CI with connection refused errors because
PostgreSQL was not configured to listen on port 7432 inside the container.

Added default values for all PostgreSQL environment variables in docker-compose.yml:
- PGPORT defaults to 7432 (ensures PostgreSQL listens on the expected port)
- PGUSER, POSTGRES_DB, POSTGRES_USER default to 'cipherstash'
- POSTGRES_PASSWORD defaults to 'password'

This ensures the container works correctly even when mise environment variables
are not properly exported to the docker compose subprocess in CI.

Author: tobyhede

tests/docker-compose.yml

@@ -6,11 +6,11 @@ services:
     ports:
       - 7432:7432
     environment:
-      - PGPORT=${POSTGRES_PORT}
-      - PGUSER=${POSTGRES_USER}
-      - POSTGRES_DB=${POSTGRES_DB}
-      - POSTGRES_USER=${POSTGRES_USER}
-      - POSTGRES_PASSWORD=${POSTGRES_PASSWORD}
+      - PGPORT=${POSTGRES_PORT:-7432}
+      - PGUSER=${POSTGRES_USER:-cipherstash}
+      - POSTGRES_DB=${POSTGRES_DB:-cipherstash}
+      - POSTGRES_USER=${POSTGRES_USER:-cipherstash}
+      - POSTGRES_PASSWORD=${POSTGRES_PASSWORD:-password}
     networks:
       - postgres
     deploy: