Add Docker containerization with automated GHCR publishing

[Copilot]

Adds Docker support and automated container image publishing to GitHub Container Registry on main branch pushes.

Implementation

Dockerfile

• Multi-stage build: SDK 9.0 (build) → ASP.NET Runtime 9.0 slim (runtime)
• Non-root user (appuser) with proper file ownership
• Health check on port 3001
• Layer caching optimized via separate dependency restore

.dockerignore

• Excludes build artifacts, IDE files, git/CI config, and documentation from build context

GitHub Actions Workflow (.github/workflows/docker-publish.yml)

• Triggers: PR validation (build only), main branch push (build + publish to ghcr.io)
• Tagging: latest, main, main-<sha>, optional semver support
• Uses GitHub Actions cache and GITHUB_TOKEN authentication

Documentation

• Usage examples for pulling from GHCR, local builds, and Docker Compose
• CI/CD section updated with deployment details
• Workflow badge added

Container Usage

docker pull ghcr.io/abeckdev/db-timetableapi-mcpserver:latest

docker run -d -p 3001:3001 \
  -e DeutscheBahnApi__ClientId="..." \
  -e DeutscheBahnApi__ApiKey="..." \
  ghcr.io/abeckdev/db-timetableapi-mcpserver:latest

Images published at ghcr.io/abeckdev/db-timetableapi-mcpserver with tags: latest, main, main-<sha>

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Add Docker container image and automate uploads to GitHub Container Registry</issue_title>
<issue_description>Task:

• Create a Dockerfile for the project in the repository that builds a container image for DB-TimetableAPI-MCPServer.
• Set up a GitHub Actions workflow that builds the Docker image whenever changes are pushed to the main branch.
• Automate the publishing of the Docker image to the GitHub Container Registry associated with this repository upon successful builds on the main branch.

Requirements:

• Ensure the Dockerfile is optimized for the app, including copy/install steps for all dependencies and build artifacts as appropriate.
• Properly tag the image (preferably using the commit SHA or semantic versioning that matches repo conventions).
• Securely use GitHub secrets for registry authentication in the workflow.
• Document the usage and workflow in the README (overview of build, push, and usage).

Acceptance Criteria:

• Repository contains a valid Dockerfile for the app.
• An automated workflow (GitHub Actions) is triggered on changes to main and uploads the container to GitHub Container Registry.
• Uploaded images are accessible and properly tagged.
• Documentation is updated to show how the container is built, tagged, and published.</issue_description>

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

• Fixes Add Docker container image and automate uploads to GitHub Container Registry #8

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.89%. Comparing base (ddd38c3) to head (71ce4c2).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main       #9   +/-   ##
=======================================
  Coverage   72.89%   72.89%           
=======================================
  Files           5        5           
  Lines         107      107           
  Branches        5        5           
=======================================
  Hits           78       78           
  Misses         29       29           

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