chore(ci): reuse variables

.drone.yml

@@ -11,9 +11,9 @@ steps:
   - name: database healthcheck
     image: &postgres 'postgres:16.3-alpine'
     environment:
-      PGUSER: postgres
-      PGPASSWORD: postgres
-      PGHOST: db
+      PGUSER: &db_user postgres
+      PGPASSWORD: &db_pass postgres
+      PGHOST: &db_host db
     commands:
       - while ! pg_isready; do sleep 1; done
 
@@ -54,9 +54,9 @@ steps:
     image: *elixir
     environment:
       MIX_ENV: test
-      POSTGRES_HOST: &db_host db
-      POSTGRES_USER: &db_user postgres
-      POSTGRES_PASS: &db_pass postgres
+      POSTGRES_HOST: *db_host
+      POSTGRES_USER: *db_user
+      POSTGRES_PASS: *db_pass
     commands:
       - apt-get update
       - apt-get install -y git make