feat: allow multiple Postal instances to run on the same host

[danquinney]

This pull request adds support for running multiple Postal instances on the same machine. Each install would have its own install and config directories, and it would be recommended that each instance have a unique Postal symlink.

git clone https://github.com/postalserver/install /opt/postal-test/install
sudo ln -s /opt/postal-test/install/bin/postal /usr/bin/postal-test

# Set the unique project name
echo postal-test > /opt/postal-test/install/docker-project-name

# Bootstrap
postal-test bootstrap postal.example.com /opt/postal-test/config