Rename serve.Dockerfile to a more general Dockerfile

[dannystaple]

The dockerfile serve.Dockerfile has multiple stages and targets now. It serves a few related purposes in the project.
The name feels incorrect and therefore, just "Dockerfile" sounds correct.

• Rename serve.Dockerfile to Dockerfile
• Update any compose references to match
• Update any README or documentation to match
• Add into .eleventyignore

Acceptance criteria

• The Dockerfile renamed.
• All compose targets still work.
• Any README or docs references to serve.Dockerfile are updated - new references are not created.
• The .eleventyignore is configured to ensure the new Dockerfile does not end up in the _site when built.
• All workflows complete and pass.

Code standards

Required before each commit

• Ensure changed files meet .editorconfig standard

Development flow

• Build the dist and serve: docker compose up --build --remove-orphans
• Build and run the staging container: docker compose up -d staging
• Run the tests: docker compose run --build test