Combine Dockerfile and .github/scripts/staging/Dockerfile

[dannystaple]

The Dockerfile .github/scripts/staging/Dockerfile and the main Dockerfile stage httpd_serve are very similar. This feels redundant.
Combine these into a single Dockerfile stage in the root Dockerfile.

• Verify that the staging Dockerfile and httpd_serve docker target are compatible. Add the necessary things to httpd_serve if needed.
• Convert the Docker compose staging service to use the httpd_serve docker stage.
• Remove the staging Dockerfile.
• Ensure all workflows still run correctly
• Update the staging test environment documentation to reference the docker compose stage as the way to correctly run it.

Acceptance criteria

• Files conform to the .editorconfig file
• GitHub workflows run and pass
• The staging Dockerfile has been removed
• The docker compose staging service runs and passes health checks
• The readme shows the docker compose method to start the staging site