[Feature] Support environment variables

Inspired by the Kubernetes spec, support environment variables.

I think run-job should remain as simple as possible, but these are normally required for many kinds of container.

[K8s format](https://kubernetes.io/docs/tasks/inject-data-application/define-environment-variable-container/)

```yaml
env:
- name: DEMO_GREETING
value: "Hello from the environment"
- name: DEMO_FAREWELL
value: "Such a sweet sorrow"
```

I'd be tempted to use the [OpenFaaS format](https://docs.openfaas.com/reference/yaml/), but the K8s style may be easier for people migrating tasks.

```yaml
environment:
DEMO_FAREWELL: "Such a sweet sorrow"
```

By running the `env` built-in shell command in `alpine:3.16`, we can see whether the inputs have been received.