Install postgresql client in image

[jenshenny]

Follow up to: #6091

This enables us to create a db readonly user for blazer via the postgresql client.

Testing

Added the app to the docker-compose file and built it

services:
  app:
    build:
      context: .
      dockerfile: Dockerfile
    environment:
      - RAILS_ENV=production
      - SECRET_KEY_BASE=secret_key_base
      - DATABASE_URL=postgresql://postgres@db:5432/rubygems_production

was able to psql and run the commands to create the db user.

Example

BEGIN;
CREATE ROLE blazer LOGIN PASSWORD 'secret';
GRANT CONNECT ON DATABASE rubygems_production TO blazer;
GRANT USAGE ON SCHEMA public TO blazer;
GRANT SELECT ON ALL TABLES IN SCHEMA public TO blazer;
ALTER DEFAULT PRIVILEGES IN SCHEMA public GRANT SELECT ON TABLES TO blazer;

rubygems_production=# \du
                                   List of roles
 Role name |                         Attributes                         | Member of 
-----------+------------------------------------------------------------+-----------
 blazer    |                                                            | {}
 postgres  | Superuser, Create role, Create DB, Replication, Bypass RLS | {}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.24%. Comparing base (27992d5) to head (3f8b472).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6097   +/-   ##
=======================================
  Coverage   97.24%   97.24%           
=======================================
  Files         476      476           
  Lines        9785     9785           
=======================================
  Hits         9515     9515           
  Misses        270      270           

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