quoting of environ is wrong

If I have an environment value that has double quotes,
```yaml
environment:
foo: 'some "quoted value"'
```
then spread will (incorrectly) do
```sh
export foo="some "quoted value""
```
which sets `foo` to `"some quoted"` and exports `foo` and `value`. This is not what was intended.

What should happen is that spread properly quotes the value. One easy way to do this would be to use `%q`, possibly. I haven't tested this.

Note this also means that all env vars need doubling of backslashes, needlessly.