docs: explain Template inheritance with Terraform modules (#8328)

* docs: explain Template inheritance with Terraform modules

* make fmt & title renaming

---------

Co-authored-by: Eric <ericpaulsen@coder.com>

docs/manifest.json

@@ -175,6 +175,11 @@
           "description": "Use docker inside containerized templates",
           "path": "./templates/docker-in-workspaces.md",
           "icon_path": "./images/icons/docker.svg"
+        },
+        {
+          "title": "Terraform Modules",
+          "description": "Reuse code across Coder templates",
+          "path": "./templates/modules.md"
         }
       ]
     },

docs/templates/modules.md

@@ -0,0 +1,89 @@
+# Template inheritance
+
+In instances where you want to reuse code across different Coder templates, such as common scripts or resource definitions, we suggest using [Terraform Modules](https://developer.hashicorp.com/terraform/language/modules).
+
+These modules can be stored externally from Coder, like in a Git repository or a Terraform registry. Below is an example of how to reference a module in your template:
+
+```hcl
+data "coder_workspace" "me" {}
+
+module "coder-base" {
+  source = "github.com/my-organization/coder-base"
+
+  # Modules take in variables and can provision infrastructure
+  vpc_name            = "devex-3"
+  subnet_tags         = { "name": data.coder_workspace.me.name }
+  code_server_version = 4.14.1
+}
+
+resource "coder_agent" "dev" {
+  # Modules can provide outputs, such as helper scripts
+  startup_script=<<EOF
+  #!/bin/sh
+  ${module.coder-base.code_server_install_command}
+  EOF
+}
+```
+
+> Learn more about [creating modules](https://developer.hashicorp.com/terraform/language/modules) and [module sources](https://developer.hashicorp.com/terraform/language/modules/sources) in the Terraform documentation.
+
+## Git authentication
+
+If you are importing a module from a private git repository, the Coder server [or provisioner](../admin/provisioners.md) needs git credentials. Since this token will only be used for cloning your repositories with modules, it is best to create a token with limited access to repositories and no extra permissions. In GitHub, you can generate a [fine-grained token](https://docs.github.com/en/rest/overview/permissions-required-for-fine-grained-personal-access-tokens?apiVersion=2022-11-28) with read only access to repos.
+
+If you are running Coder on a VM, make sure you have `git` installed and the `coder` user has access to the following files
+
+```sh
+# /home/coder/.gitconfig
+[credential]
+  helper = store
+```
+
+```sh
+# /home/coder/.gitconfig
+
+# GitHub example:
+https://your-github-username:your-github-pat@github.com
+```
+
+If you are running Coder on Docker or Kubernetes, `git` is pre-installed in the Coder image. However, you still need to mount credentials. This can be done via a Docker volume mount or Kubernetes secrets.
+
+### Passing git credentials in Kubernetes
+
+First, create a `.gitconfig` and `.git-credentials` file on your local machine. You may want to do this in a temporary directory to avoid conflicting with your own git credentials.
+
+Next, create the secret in Kubernetes. Be sure to do this in the same namespace that Coder is installed in.
+
+```sh
+export NAMESPACE=coder
+kubectl apply -f - <<EOF
+apiVersion: v1
+kind: Secret
+metadata:
+  name: git-secrets
+  namespace: $NAMESPACE
+type: Opaque
+data:
+  .gitconfig: $(cat .gitconfig | base64 | tr -d '\n')
+  .git-credentials: $(cat .git-credentials | base64 | tr -d '\n')
+EOF
+```
+
+Then, modify Coder's Helm values to mount the secret.
+
+```yaml
+coder:
+  volumes:
+    - name: git-secrets
+      secret:
+        secretName: git-secrets
+  volumeMounts:
+    - name: git-secrets
+      mountPath: "/home/coder/.gitconfig"
+      subPath: .gitconfig
+      readOnly: true
+    - name: git-secrets
+      mountPath: "/home/coder/.git-credentials"
+      subPath: .git-credentials
+      readOnly: true
+```