Continuous deployment implementation

[tobymurray]

As of #29, this project should automatically build. The next step is to automatically deploy.

Mentioned here:

Internally, we currently use rsync to deploy builds to our infrastructure. We’ve just set up a staging server (map-int.airgradient.com) and for the near future, we’ll be deploying manually. The flow is still evolving and a bit rough around the edges, so we’re keeping it flexible for now.

I think it should still be possible to deploy with rsync from GitHub. There's not too much I'd be able to do as it would require an admin on AirGradient's machine but:

1. Generate an SSH key pair (anywhere, can be a dev machine)
   • ssh-keygen -t ed25519 -C "github-actions" -f ./github-actions-key
2. Add the private key to GitHub Actions as a secret (through GitHub UI)
3. Add the public key to the deployment host
   • Add github-actions-key.pub to ~/.ssh/authorized_keys for the relevant user on the machine
4. Add the GitHub action to actually deploy

Ensure the remote server accepts the public key, then we can add a GitHub action to rsync the build artifact over.

Couple extra quirks:

• we should not rsync over the existing app to avoid breaking the live site. We can use unique identifiers on the artifact to deploy to a folder and then update a symlink server side to "deploy" once the rsync is complete
• There is a security consideration to giving GitHub an SSH key for the host machine. This is not my specialty, but some things that might be worth doing:
  • add a new user on the host specifically for deploys (with access to the deploy directory)
  • put command restrictions on the public key in the authorized_keys file
  • probably some other stuff