Linters for Envbuilder aren't catching some common concerns that we have

Suggestion: It's best practice for exported functions to have documentation (`// EnvWithBuildSecretPrefix ...`). It's part of the original Go linter (https://pkg.go.dev/github.com/golang/lint) and [CodeReviewComments](https://go.dev/wiki/CodeReviewComments#doc-comments)/[Effective Go](https://go.dev/doc/effective_go#commentary), so a bit surprising that it's not caught.

```
❯ golint ./...
envbuilder.go:43:2: a blank import should be only in a main or test package, or have a comment justifying it
devcontainer/devcontainer.go:34:6: exported type Spec should have comment or be unexported
devcontainer/devcontainer.go:50:6: exported type LifecycleScripts should have comment or be unexported
devcontainer/devcontainer.go:57:6: exported type BuildSpec should have comment or be unexported
devcontainer/devcontainer.go:78:1: exported function SubstituteVars should have comment or be unexported
devcontainer/script.go:16:6: exported type LifecycleScript should have comment or be unexported
devcontainer/script.go:21:1: exported method LifecycleScript.IsEmpty should have comment or be unexported
```

It seems `golint` is marked deprecated in `golangci-lint` though, so I guess the alternative linters aren't enforcing this rule. 😔 I always found it slightly annoying but ultimately worth the annoyance 😄.

_Originally posted by @mafredri in https://github.com/coder/envbuilder/pull/391#discussion_r1818991909_

---

There seem to be some linting rules that we'd value here. Two of them are:
* Tests should run in parallel when they can
* Exported functions should be documented

There may be others. It would be worth checking coder/coder's linting config to see what else we're missing. We should add the linters we deem important and ensure the project conforms to them.

Start by reviewing the linting configuration and comparing it with coder/coder's configuration, then run the current linter against envbuilder.go, devcontainer/devcontainer.go, and devcontainer/script.go. Add the agreed rules for exported documentation and parallelizable tests, address the resulting violations, and verify the project lint and test commands pass.

索引モデルが issue の本文から書いたものです。