Add latest tag to Docker image builds for main branch

[Copilot]

Overview

Fixes the missing latest Docker image tag by updating the CD workflow to automatically create it when building from the main branch.

Problem

The repository already had complete Docker image build infrastructure and documentation, but the CD workflow wasn't creating a latest tag. Users following the README documentation would find that ghcr.io/chrisw-dev/golang-mock-oauth2-server:latest didn't exist, forcing them to clone and build locally or use SHA-based tags.

Solution

Added a single line to the Docker metadata action configuration in .github/workflows/cd.yml:

type=raw,value=latest,enable={{is_default_branch}}

This ensures that builds from the main branch automatically create a latest tag alongside the existing branch and SHA tags.

Result

Users can now pull and use the pre-built image directly from GitHub Container Registry without needing to build it themselves:

# Pull the latest image
docker pull ghcr.io/chrisw-dev/golang-mock-oauth2-server:latest

# Run it directly
docker run -p 8080:8080 ghcr.io/chrisw-dev/golang-mock-oauth2-server:latest

Docker image tags now created:

• Main branch pushes: main, sha-XXXXXXX, latest (new)
• Version tag pushes (e.g., v1.0.0): 1.0.0, 1.0, sha-XXXXXXX

This aligns the actual workflow behavior with what's already documented in the README.

Testing

• ✅ All existing Go tests pass
• ✅ YAML syntax validated
• ✅ No breaking changes
• ✅ Minimal, surgical modification (1 line added)

Closes #[issue_number]

Original prompt

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This section details on the original issue you should resolve

<issue_title>[FEATURE] Create docker image and share</issue_title>
<issue_description>## Problem Statement
When using this tool I need to clone the repo and then build a docker image. I'd like to just be abe to use an existing image hosted in a registry

Proposed Solution

Create a container image and build it as part of the release process for this repo. It should be stored in an appropriate registry so that the image can be used in other projects.

Alternative Solutions

No other options considered

Implementation Details

none

Additional Context

n/a</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #1

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