Update for use on GitHub Codespaces

[hummusonrails]

In this PR, the devContainer.json file is updated to be used with Codespaces and the following additions are made:

• Fetch a pre-built binary of the node-template from Docker Hub
• Make the binary available in /usr/local/bin
• Add a Dockerfile.build so that the binary can be rebuilt for new updates and include instructions on how to publish it to Docker Hub
• Updates the README to include instructions and a link to Codespaces (note: the badge link won't work until this is published)