feat(ci): Add integration tests with testcontainers

[ATMackay]

• Add integration test support using testcontainers

[Copilot]

Pull Request Overview

Add integration tests powered by Testcontainers to spin up Postgres and the checkout app in CI and locally, and wire them into the build.

• Introduce a reusable Testcontainers stack (network, Postgres, app) with logging support.
• Add an integration test to add inventory items and verify pricing.
• Update Makefile, Dockerfile, and GitHub Actions workflow to build and run integration tests.

Reviewed Changes

Copilot reviewed 7 out of 8 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
integration/utils.go	New helpers to create a Docker network, run Postgres and the app containers, and build a test client.
integration/add_inventory_test.go	New integration test using the Testcontainers stack to add items and verify prices.
integration/concurrent_write_test.go	Commented-out scaffold for a future concurrent write test (no functional changes).
go.mod	Adds testcontainers and updates dependencies to support integration tests.
Makefile	Adds a dedicated integration test target with tag and timeout.
Dockerfile	Passes build metadata as args into the make build during image build.
.github/workflows/go.yml	Adds an integration-test job running the new tests on ubuntu-latest.

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