Terraform Infrastructure As Code

**Context**: 
Terraform is an infrastructure as code (IaC) tool that allows users to define and manage cloud and on-premises resources using human-readable configuration files. The goal is to create a scalable, secure, and efficient infrastructure for a web application using Terraform.

**Detailed Instructions**: 
Design a Terraform configuration to deploy a web application on Amazon Web Services (AWS). The infrastructure should include:
1. A Virtual Private Cloud (VPC) with a public subnet and a private subnet.
2. An Application Load Balancer (ALB) to distribute traffic to the web servers.
3. An Auto Scaling group with two EC2 instances running a Linux operating system and a Docker container with the web application.
4. A Relational Database Service (RDS) instance for data storage.
5. Security groups to control inbound and outbound traffic.
6. A key pair for secure access to the EC2 instances.

The configuration should be modular, reusable, and follow best practices for security and cost optimization. Use Terraform 1.3 or later and AWS provider version 4.0 or later.

**Output Format**: 
The output should be a Terraform configuration file (`.tf` file) that includes all the necessary resources and modules to deploy the infrastructure. The file should be well-structured, readable, and include comments to explain each section.

**Examples**: 
For reference, the following Terraform configuration snippet demonstrates how to create an EC2 instance:
```terraform
resource "aws_instance" "example" {
  ami           = "ami-abc123"
  instance_type = "t2.micro"
  vpc_security_group_ids = [aws_security_group.example.id]
  key_name               = "example-key"
}
```
Similarly, the following snippet shows how to create an RDS instance:
```terraform
resource "aws_db_instance" "example" {
  allocated_storage    = 20
  engine               = "postgres"
  engine_version       = "13.4"
  instance_class       = "db.t2.micro"
  name                 = "exampledb"
  username             = "exampleuser"
  password             = "examplepassword"
  vpc_security_group_ids = [aws_security_group.example.id]
}
```
These examples illustrate the structure and syntax of Terraform configuration files, but the actual output should include all the required resources and modules to deploy the entire infrastructure.