Introduce Vagrantfile for a test environment

Vagrantfile

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+# -*- mode: ruby -*-
+# vi: set ft=ruby :
+
+Vagrant.configure("2") do |config|
+  config.vm.box = "debian/buster64"
+
+  config.vm.synced_folder "./", "/vagrant", type: "rsync"
+
+  # Export postfix port 25 (guest) to port 2525 (host)
+  # This is useful for throwing swaks and other tools against postfix
+  config.vm.network "forwarded_port", guest: 25, host: 2525
+
+  config.vm.provision "shell", inline: <<-SHELL
+    # Give that vagrant box an apparently good FQDN
+    echo "local-vagrant.example.com" > /etc/hostname
+    echo "local-vagrant.example.com" > /etc/mailname
+    echo "127.0.1.2 local-vagrant.example.com local-vagrant" > /etc/hosts
+    hostnamectl set-hostname local-vagrant.example.com
+
+    # Basic preparations
+    apt-get update
+
+    # Install postfix
+    DEBIAN_FRONTEND=noninteractive apt-get install -y postfix
+
+    # Add milter to configuration
+    postconf smtpd_milters=inet:127.0.0.1:7777
+    postconf non_smtpd_milters=inet:127.0.0.1:7777
+
+    # Enable and launch postfix
+    systemctl enable postfix.service
+    systemctl start postfix.service
+
+    # Install swaks
+    DEBIAN_FRONTEND=noninteractive apt-get install -y swaks
+
+    # That's it
+    echo "That's it. We're set up. Enjoy your test environment."
+  SHELL
+end