Example: Using `devcontainer.json` to influence a Coder (Terraform) template

> [!NOTE]
> I'm not sure how realistic this is, or whether it should be out of scope envbuilder as it's likely a Terraform provider/Coder feature. However, this is a somewhat common use case. It may not be a good fit for envbuilder v1.0 but worth discussing

Since envbuilder is a container image, it cannot influence the underlying infrastructure it can be provisioned on. However some Dev Container features do have support for additional infrastructure layers. Can we provide an example of how a Coder template can read a devcontainer.json and then influence the template? We have some customers doing this today with a custom `coder.yaml`

## Example: Kubernetes Volume Mounts

```hcl
# The jank approach (pseudocode, untested)
data "http" "devcontainer_contents" {
url = "${var.git_repo}/${var.git_branch}/.devcontainer/devcontainer.json"

request_headers = {
Accept = "application/vnd.github.v3.raw"
}
}

locals {
# Attempt to parse mounts from the devcontainer JSON
parsed_mounts = try(jsondecode(data.http.devcontainer_contents.body).mounts, [])

# Use a default mount if no mounts are specified or if the mounts key doesn't exist
mounts = length(local.parsed_mounts) > 0 ? local.parsed_mounts : ["source=workspace,target=/workspaces,type=volume"]

# Extracting target paths for PVC creation and Pod mounts
target_paths = [for m in local.mounts : split(",", m)[1]]
target_path_names = { for m in local.target_paths : split("=", m)[1] => split("=", m)[1] }
}

resource "kubernetes_persistent_volume_claim" "pvc" {
for_each = local.target_path_names

metadata {
name = "pvc-${replace(each.value, "/", "-")}"
}

spec {
access_modes = ["ReadWriteOnce"]
resources {
requests = {
storage = "1Gi"
}
}
}
}

resource "kubernetes_pod" "example" {
metadata {
name = "example-pod"
}

spec {
container {
image = "nginx"
name = "example"

volume_mount {
for_each = local.target_path_names
name = "pvc-${replace(each.value, "/", "-")}"
mount_path = each.value
}
}

volume {
for_each = local.target_path_names
name = "pvc-${replace(each.value, "/", "-")}"

persistent_volume_claim {
claim_name = "pvc-${replace(each.value, "/", "-")}"
}
}
}
}
image = "nginx"
name = "example"

volume_mount {
for_each = local.target_path_names
name = "pvc-${replace(each.value, "/", "-")}"
mount_path = each.value
}
}

volume {
for_each = local.target_path_names
name = "pvc-${replace(each.value, "/", "-")}"

persistent_volume_claim {
claim_name = "pvc-${replace(each.value, "/", "-")}"
}
}
}
}
```

## Option 2: TF Provider

The Coder server reads the devcontainer.json and passes data via a Terraform data source. This is related to https://github.com/coder/coder/issues/8462.

## Option 3: envbuilder + docker on a VM

If envbuilder runs on a VM, it could realistically do significantly more such as create Docker volumes as well as run a container.

The issue names no repository files or tests; begin by reviewing the proposed Coder Terraform template and the three options described. A contribution would first need a maintainer decision on scope, then a documented, validated example with a clear location and verification path.

Written by the indexing model from the issue text.