Part of the Template Builder wizard PR stack.
## Problem
The kubernetes base template used Terraform `variable` blocks and
`var.*` references for `use_kubeconfig` and `namespace`, but the
composed tar bundle never included a `.tfvars` file. This caused
`terraform plan` to fail with "required template variables need values:
namespace".
## Fix
Base templates now use Go template variables (`{{ .Variables.* }}`) just
like module templates do. Values are validated, HCL-quoted, and rendered
directly into the output HCL.
Also adds explicit "variable is required" validation to both
`mergeBaseVariables` and `mergeModuleVariables`, replacing the previous
reliance on `missingkey=error` at render time for clearer error
messages.
---
> [!NOTE]
> Generated by Coder Agents on behalf of @jeremyruppel
Add the bundled `exampleID -> OS` Go map for Docker, Kubernetes, and AWS
EC2 Linux base templates. Create `.tf.tmpl` Go template files for each
within `coderd/templatebuilder/bases/`, along with `BaseRenderContext`
and `RenderBaseTemplate` rendering helpers.
The `.tf.tmpl` files are independent copies of the example templates
with module blocks (code-server, jetbrains) removed, since the template
builder composes modules separately into `modules.tf`. When
`ImageOptions` is provided, the container image field references the
Terraform parameter; otherwise it uses the hardcoded value via Go
template whitespace control (`{{-`).
Golden file snapshot tests verify rendered output stability with an
`-update` flag for regeneration.
Depends on #25909
> [!NOTE]
> This PR was authored by Coder Agents on behalf of @jeremyruppel.
Jeremy Ruppel