Add Docker Compose setup for self-hosted Shlink on Coolify

.env.example

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+# Your short URL domain (e.g. s.example.com)
+DEFAULT_DOMAIN=s.example.com
+
+# Set to false if running without TLS termination
+IS_HTTPS_ENABLED=true
+
+# Database
+DB_NAME=shlink
+DB_USER=shlink
+DB_PASSWORD=change_me_strong_password
+
+# Generate a UUID or random string for the initial API key
+# e.g.: openssl rand -hex 32
+INITIAL_API_KEY=change_me_api_key
+
+# Timezone (https://www.php.net/manual/en/timezones.php)
+TIMEZONE=UTC