Improve CI test run time and inconsistency (#5418)

**Story card:**
[sc-12505](https://app.shortcut.com/simpledotorg/story/12505)

The CI test job is working inconsistently and takes a variable amount of
time to complete each run.

The number of parallelisms has been increased to 32, and each parallel
run will receive a Docker-based dedicated PostgreSQL instance.

.semaphore/semaphore.yml

@@ -28,8 +28,6 @@ blocks:
   - name: Tests
     task:
       env_vars:
-        - name: DATABASE_URL
-          value: "postgresql://postgres:@0.0.0.0:5432/myapp_test"
         - name: RAILS_ENV
           value: "test"
         - name: TB_RSPEC_OPTIONS
@@ -44,9 +42,12 @@ blocks:
             - bundle install
             - "bundle exec standardrb"
         - name: RSpec
-          parallelism: 8
+          parallelism: 32
           commands:
             - checkout
+            - docker run -d --name postgres14-$SEMAPHORE_JOB_INDEX -p 543$SEMAPHORE_JOB_INDEX:5432 -e POSTGRES_HOST_AUTH_METHOD=trust postgres:14
+            - export DATABASE_URL=postgresql://postgres@localhost:543$SEMAPHORE_JOB_INDEX
+            - export TEST_ENV_NUMBER=$SEMAPHORE_JOB_INDEX
             - script/semaphore_setup
             - rspec_booster --job $SEMAPHORE_JOB_INDEX/$SEMAPHORE_JOB_COUNT
       epilogue: