Change default network to local

[leighmcculloch]

I think the default network of the image should change from testnet to local.

This will be a breaking change.

Why:

The quickstart image is primarily intended for use during development and testing. It's defaults should operate to serve that purpose. When folks are testing they typically test either against the public test network or a local network. When testing against the public test network the easiest path to access is to approach is to communicate directly with the SDF hosted testnet RPC at https://soroban-testnet.stellar.org or the SDF hosted testnet Horizon at https://horizon-testnet.stellar.org. The quickstart image only adds value to setup when running a local network.

Today the image when started without any other parameters defaults connecting to testnet. This means for the default common case a developer needs to read the docs and pass additional parameters.

The stellar-cli has a stellar container start command that defaults already to starting the container in local mode for the same reason.

Impact:

For anyone using this image without parameters to connect to testnet, they will see the container instead connect to local and will need to add a parameter --testnet.

cc @stellar/devx @janewang