Fix compatibility issue with Docker Compose 2

[ollietreend]

The Docker Compose project has moved from v1 to v2.

For now, it's still possible to opt-out of v2 and revert to v1 when using Docker for Mac. However this option will likely go away at some point. It's therefore a good idea to ensure GOV.UK Docker works with both versions.

As far as I can see, the only compatibility issue that existed is that the nginx container attaches to the 'default' network. As I understand it, the network called default is no longer implicitly created with Docker Compose v2, so it needs to be defined in the docker compose file.

For more info, see: docker/compose#9015

It's not clear when this change happened as it's not documented as a known change: https://docs.docker.com/compose/compose-file/compose-versioning/

Demonstration of compatibility issue

The scripts bin/replicate-mysql.sh and bin/replicate-postgres.sh both run the command:

govuk-docker config

When running with Docker Compose v1, this command outputs a YAML document describing the current project config.

With Docker Compose v2, the command fails with an error:

$ govuk-docker config
docker-compose -f [...] config
service "nginx-proxy" refers to undefined network default: invalid compose project

However when the default network is explicitly defined in the main docker-compose.yml file, both v1 and v2 run correctly without error.

With this change applied, I'm now able to successfully run bin/replicate-mysql.sh and bin/replicate-postgres.sh regardless of the Docker Compose version I'm using.

[ollietreend]

This PR effectively makes the troubleshooting content added in #506 redundant – so I've removed it as part of this change.

[ChrisBAshton]

Just tried it locally with Content Publisher and it worked - nice job! ⭐