tests: Build and run examples as end to end tests

[euanh]

Motivation

For each pull request, the CI runs a number of test suites:

• Soundness: these are static checks of formatting, language and so on
• Unit tests: these check individual library functions in isolation
• Integration tests: these check individual registry library functions against a test registry instance
• Swift 6 Language Mode: this checks that the project compiles with Swift 6

These are all unit or integration tests; there are no complete system tests to exercise the build-container-image plugin, or that the examples work. It is difficult to test the build-container-image plugin in XCTest, but with GitHub Actions we can work through all the steps of building and deploying one of the examples, including using a Swift SDK. This will cover all the major parts of the project and ensure that the examples continue to work.

Modifications

This commit adds a new GitHub Actions test which:

• Installs the static Linux SDK
• Uses it to build an example service
• Publishes the example image to a registry running in a service container
• Deploys the image using Docker provided by the GitHub Actions runner
• Verifies that the example service works

Result

An end-to-end build and deployment of one of the examples will be attempted for every pull request.

Test Plan

This commit adds a new test, which passes. All existing tests continue to pass.