Terraform init hooks tutorial (#1376)

Co-authored-by: Brian Rinaldi <[email protected]>

content/en/references/init-hooks.md

@@ -33,7 +33,8 @@ In these directories, you can put either executable shell scripts or Python prog
 All except `boot.d` will be run in the same Python interpreter as LocalStack, which gives additional ways of configuring/extending LocalStack.
 You can also use subdirectories to organize your init scripts.
 
-Currently, known script extensions are `.sh`, and `.py`.
+Currently, known script extensions are `.sh` and `.py`.
+Additionally, with the installation of the `localstack-extension-terraform-init` [extension]({{<ref "user-guide/extensions/">}}), `.tf` files can also be supported.
 Shell scripts have to be executable, and have to have a [shebang](https://en.wikipedia.org/wiki/Shebang_(Unix)) (usually `#!/bin/bash`).
 
 A script can be in one of four states: `UNKNOWN`, `RUNNING`, `SUCCESSFUL`, `ERROR`.
@@ -154,6 +155,79 @@ DOCKER_FLAGS='-v /path/to/init-aws.sh:/etc/localstack/init/ready.d/init-aws.sh'
 
 Another use for init hooks can be seen when [adding custom TLS certificates to LocalStack]({{< ref "custom-tls-certificates#custom-tls-certificates-with-init-hooks" >}}).
 
+### Terraform configuration files as init hooks
+
+Running Terraform configuration files as init hooks requires the installation of a special extension.
+For more information on how to manage [LocalStack extensions]({{< ref "user-guide/extensions/" >}}), please refer to the dedicated documentation page,
+and for more details on running init hooks in development mode, you can check out the [extension repository description](https://github.com/localstack/localstack-extensions/tree/main/terraform-init).
+
+##### Usage
+
+Start LocalStack with **`EXTENSION_AUTO_INSTALL="localstack-extension-terraform-init"`**.
+Mount a **`main.tf`** file into **`/etc/localstack/init/ready.d`**
+When LocalStack starts up, it will install the extension, which in turn installs Terraform and [`tflocal`](https://github.com/localstack/terraform-local) into the container.
+If one of the init stage directories contain a `main.tf` file, the extension will run `tflocal init` and `tflocal apply` on that directory.
+
+##### Example
+
+main.tf:
+
+```terraform
+resource "aws_s3_bucket" "example" {
+bucket = "my-tf-test-bucket"
+
+tags = {
+Name        = "My bucket"
+Environment = "Dev"
+  }
+}
+```
+
+Start LocalStack Pro with mounted main.tf:
+
+{{< tabpane >}}
+{{< tab header="docker-compose.yml" lang="yml" >}}
+version: "3.8"
+
+services:
+  localstack:
+    container_name: "localstack-main"
+    image: localstack/localstack-pro  # required for Pro
+    ports:
+      - "127.0.0.1:4566:4566"            # LocalStack Gateway
+    environment:
+# Activate LocalStack Pro: https://docs.localstack.cloud/getting-started/auth-token/
+      - LOCALSTACK_AUTH_TOKEN=${LOCALSTACK_AUTH_TOKEN:?}
+      - EXTENSION_AUTO_LOAD=localstack-extension-terraform-init
+    volumes:
+# you could also place your main.tf in `./ready.d` and set "./ready.d:/etc/localstack/init/ready.d"
+      - "./main.tf:/etc/localstack/init/ready.d/main.tf"
+      - "./volume:/var/lib/localstack"
+      - "/var/run/docker.sock:/var/run/docker.sock"
+{{< /tab >}}
+
+{{< tab header="LocalStack CLI" lang="bash" >}}
+  localstack start \
+  -e EXTENSION_AUTO_INSTALL="localstack-extension-terraform-init" \
+  -v ./main.tf:/etc/localstack/init/ready.d/main.tf
+{{< /tab >}}
+{{< /tabpane >}}
+
+You can wait for LocalStack to complete the startup process, and then print the created S3 bucket:
+{{< command >}}
+localstack wait && awslocal s3 ls
+{{< / command >}}
+
+The logs should show something like:
+
+```bash
+2024-06-26T20:36:19.946  INFO --- [ady_monitor)] l.extension                : Applying terraform project from file /etc/localstack/init/ready.d/main.tf
+2024-06-26T20:36:19.946 DEBUG --- [ady_monitor)] localstack.utils.run       : Executing command: ['tflocal', '-chdir=/etc/localstack/init/ready.d', 'init', '-input=false']
+2024-06-26T20:36:26.864 DEBUG --- [ady_monitor)] localstack.utils.run       : Executing command: ['tflocal', '-chdir=/etc/localstack/init/ready.d', 'apply', '-auto-approve']
+```
+
+For a more complex demo project, on how to use Terraform init hooks for your testing environments, you can check out [this example]({{< ref "tutorials/using-terraform-with-testcontainers-and-localstack/" >}}) in the Tutorials section.
+
 ## Troubleshooting
 
 If you are having issues with your initialization hooks not being executed, please perform the following checks: