feat: make inner container name configurable via CODER_CVM_CONTAINER_NAME

## Summary

The inner container name is currently hardcoded to `workspace_cvm` and cannot be changed. There is already an environment variable constant `CODER_CVM_CONTAINER_NAME` defined in the code, but it is never used to override the container name.

## Current Behavior

The container name is hardcoded:
```go
InnerContainerName = "workspace_cvm"
```

And used directly when creating the container:
```go
containerID, err := dockerutil.CreateContainer(ctx, client, &dockerutil.ContainerConfig{
// ...
Name: InnerContainerName,
// ...
})
```

## Proposed Solution

Leverage the existing `CODER_CVM_CONTAINER_NAME` environment variable to allow users to configure the inner container name. If not set, default to the current `workspace_cvm`.

## Use Case

Users may want to customize the container name for organizational purposes, logging clarity, or to avoid conflicts in environments where multiple envbox instances are running.

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_Issue created on behalf of a customer request._