Update Dockerfile and BATS tests for MailHog

[tobybellwood]

This pull request introduces several improvements to the build, test, and development workflow for the MailHog Docker image. The most significant changes include updating the Docker base images, enhancing the GitHub Actions workflow to use more secure and up-to-date actions with pinned SHAs, adding comprehensive BATS-based tests for image validation, and providing a new Makefile to streamline local development and testing.

Build and Test Workflow Improvements:

• Updated the GitHub Actions workflow in .github/workflows/build_and_test.yml to use pinned SHA references for all actions, improving security and reproducibility. Also upgraded to the latest major versions of key Docker actions. [1] [2] [3] [4]
• Enhanced the workflow to set and use a single image tag, perform find-and-replace in example files, and install required tools (bats-core, pygmy, dockerize) via Homebrew.
• Added steps to pull required images and run new BATS-based image structure and runtime tests as part of the CI pipeline.

Testing Enhancements:

• Introduced tests/image_structure.bats, a BATS test suite that verifies the presence, executability, and configuration (ports, entrypoint, working directory) of the MailHog binary in the Docker image.

Development Tooling:

• Added a Makefile that provides targets for building the Docker image, running all or specific BATS tests, opening a shell in the container, and cleaning up local images. This simplifies local development and testing workflows.

Docker Image Updates:

• Updated the Dockerfile to use newer base images: golang:1.25-alpine3.23 for building and alpine:3.23 for the runtime image, ensuring up-to-date dependencies and security patches.

[Copilot]

Pull request overview

Adds automated structure and runtime verification for the MailHog Docker image (including Lagoon/ssmtp email-flow checks), and wires those tests into local workflows and CI while updating the build base images.

Changes:

• Add BATS-based image structure tests and runtime SMTP/HTTP/API + Lagoon ssmtp integration tests.
• Add a Makefile to build the image and run the BATS suites locally.
• Update the Dockerfile base images and refresh/pin GitHub Actions used in the build/test workflow.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
tests/runtime.bats	New end-to-end runtime tests using a real MailHog container + Lagoon PHP ssmtp flows.
tests/image_structure.bats	New lightweight image metadata/binary/entrypoint/ports assertions.
Makefile	Local developer entrypoints for building and running the BATS suites.
Dockerfile	Updates builder/runtime base images for the MailHog image build.
.github/workflows/build_and_test.yml	Pins/updates actions and runs the new BATS tests in CI.

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