Due to a typo, substitution would not work if the given
environment-variable was set.
Given the following docker compose file;
```yaml
version: "3.7"
services:
app:
image: nginx:${version:-latest}
```
Deploying a stack with `$version` set would ignore the `$version`
environment variable, and use the default value instead;
```bash
version=alpine docker stack deploy -c docker-compose.yml foobar
Creating network foobar_default
Creating service foobar_app
docker service ls
ID NAME MODE REPLICAS IMAGE PORTS
rskkjxe6sm0w foobar_app replicated 1/1 nginx:latest
```
This patch also fixes "soft default" not detecting empty environment variables,
only non-set environment variables.
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
ec3daea021