Get production server to automatically deploy new version when `master` changes

[amcewen]

The production server is running off the master branch, but currently to update it you need to log in and then run these steps:

1. cd optimism
2. git pull
3. docker-compose build
4. docker-compose up

We should have it auto-deploy when there are new commits to master.

[amcewen]

Set up a new Github Action to deploy this whenever something is pushed to master.

There's probably a better way to do it (running docker-compose build in the action and then pushing the containers over to the site, for example), but given there are just the four steps listed in the description at present I've gone with a short ssh script.

Needed to set up new Action Secrets in the Secrets section of Settings for the repo, to hold REMOTE_HOST_PROD, REMOTE_USER_PROD and SSH_PRIVATE_KEY_PROD

SSH_PRIVATE_KEY_PROD contains an ssh key that I created just for this:

1. ssh-keygen -f /path/to/somewhere/suitable/id_rsa with no passphrase
2. ssh-copy-id -i /path/to/somewhere/suitable/id_rsa.pub PRODUCTION_USER@PRODUCTION_HOST to allow the key access to the server
3. Copy the contents of /path/to/somewhere/suitable/id_rsa into SSH_PRIVATE_KEY_PROD

You can run the action manually by going to https://github.com/DoESLiverpool/optimism/actions/workflows/deploy-to-production.yaml but it should automatically deploy when something is pushed to master. It seemed to work fine when I just tried it manually, so now to test with a new push to master...

[amcewen]

docker-compose restart just restarts the existing containers, and doesn't pick up any new ones, so I had to switch that to docker-compose down followed by docker-compose up -d.

But now the site auto deploys when anyone pushes new changes to master!!! 🎉